Honestly, almost anything can work. There is surprisingly little character progression in the game, thanks to leveled enemies and all that. And this is even more true after CP 2.0 when they buffed the base stats of all characters and nerfed CP to the ground.

Sets, top sets, do add a lot of raw power to you, but they are not needed for just clearing content. In vet dungeons good gear is welcome, but if you do them on normal, just use what feels cool and you'll be fine. Player skill progression is lot bigger component than raw power or even skills used.

An interesting and personal build that is fun to play is far more important than chasing meta. That stuff matters only for leader boards and for chasing achievements in Veteran Content.

As for the skills and abilities... Well almost any passive is good idea to unlock, most offer at least some benefit. As to what skills to place on your bar... That depends lot more on what you are thinking of doing with your play time than what is "optimal." Are you going questing? Doing pilfering and other thieves guild stuff? Or maybe Dark Brotherhood and knifing people in the dark is what you are after.

And as a solo player you need to remember that you can't rely on others, so you need to tank and heal and deal damage on your own. And you need to manage mobs. ESO has lot of content with crap ton of mobs. So a strong spammable AOE is really handy. Of course, as a Nightblade you can just cloak past all the pesky meaningless mobs, but that wont be an option of you are trying to farm them for something. Like an antiquity lead or a piece of a collectable. In solo play you will probably need to switch some skills in and out depending on what content you are doing.

Dual Wield is prolly the straightforward choice for front bar, and it offers Steel Tornado for that AOE. But Two handed also a good option, especially with backbar Bow. Two handed gives you a lot easier access to Major Brutality, which is the most important buff to keep up. NB has really lousy options for that, and Dual Wield requires target to work. You can use potions, but that gets expensive real quick.Forward Momentum gives you a heal, Major Brutality, and a way to deal with all the pesky snares that litter the game. In a group setting, dual wield is prolly the better option, but for a solo player, it's hard to beat the versatility and utility that Two Handed offers.

With a backbar bow, you can slot Bombard or Acid Spray for that AOE to deal with large backs of mobs, and use two hander front bar as your single target damage. Acid Spray gives you a decent DOT but I prefer Bombard for the immobilize. It makes managing large crowds of foes lot easier. It's not optimal DPS wise, but as a solo player it is not always about doing just damage. Caltrops and Hail of Arrow are far more finicky to use than Arrow Spray and it's morphs, and do damage over too long a time to be of use in most solo content. At least that is my experience. You want to hit hard and hit fast, immobilize and stun enemies and kill them quick and then fade back to the shadows. In a tough boss fight though... Hail of Arrows especially will come in handy. But it depends on the mechanics. If the boss moves a lot and you don't have a tank to hold it in place, then that skill is kinda wasted.

And if you go with a bow backbar, then Poison arrow is really hard to skip. I use Venom arrow on the back bar for utility. The interrupt it offers is too good to pass by. Especially when running solo. Personally, for me at least, Resolving Vigor is also a must on the Bow bar.

AS for the rest, well there are a lot of options. It really depends on your personal play style. A lot of the nightblade kills, at least in my eyes, are too finicky to be of any real use in solo PVE. They are much more useful in PVP and in group based play. Here are my thoughts on the ones you listed:

Expert Hunter -> Camo Hunter - I would not allocate a slot for this, because the stealth detection is meaningless in PVE and I don't need the Major Savagery for most PVE content, and when I do, I can use a potion for that. Even Trap Beast makes more sense. Though I seldom have space for it. Might slot for a tough boss fight though. Trap Beast that is, not Expert Hunter. Potions take care of that.

Veiled Strike -> Surprise Attack - Can work as a decent single target spammbale. I used to go for Concealed Weapon even on my Stamblade, simply for the cloaked speed bonus, But these days there are other options for getting cloaked speed up (which do not involve becoming a vampire) so I might give this a shot as my spammable. Although... In PVP I still kinda would want to have that speed buff. Even though I would never use the skill for anything, It would just sit on my bar and provide the speed buff.

Whirlwind -> Steel Tornado - The only real downside is that you need to go Dual Wield for this, which is fine, especially in group play. But I prefer having easy access to Major Brutality that Two handed offers, so it's not a choice for me.

Relentless Focus -> Grim Focus - It can hit like a truck but requires along set up time. And most of the time in Solo PVE you are fighting things that just do not require that sort of burst. If you run as a dedicated DPs in a group... Then I suppose it's a different story. I might use it on a flex slot when soloing a world boss. But that depends on the boss and what mechanics are involved. I usually drop cloak when I need to face a tough fight and slot skills like these, but for most situations it's just too slow and finicky to set up to be wroth it.

Assassins Blade -> Killer Blade - Not the best execute in the game, but it's fine. And it does synergize with class passives so... Even on a Two handed weapon bar, I slot this over Reverse Slash.

Death Stroke -> Incapacitating Strike - Not the best Ultimate but not the worst. And again it synergizes with class passives so It's a good choice.

Back Bar(when swap works correctly):

Caltrops -> Razor Caltrops - Too slow to be of use in solo play. And fits more a tanky sort of playstyle wher you stand your ground. Stamblades are all about he roly poly, so the fight wont stay in one spot..

Blood Alter -> Overlowing Alter - No point for solo play. You can't activate it's synergy which is kinda it's main point. Plus the health cost is steep - fits more a tank than a solo nigh blade running around in medium armor. You can, of course, go for a tankblade, but that means all heavy and a totally different play style. Leech tanks used to be a big deal, don't know how they fare these days though.

Summon Shade -> Dark Shade - Not much use in most solo situations. Though in some boss fights will allow you unusual tactical options. But other than that, it has little use in solo play. Or PVE in general.

Rapid Maneuver -> Charging Maneuver - Just build speed into your build. You really don't need this for solo play. The only reason to slot it was to increase mount movement, but that is now handled automatically by the Continuous Attack passive. Waste of a slot. If you want to go fast, just add speed to your build itself.

Vigor -> Resolving Vigor - For a stamblade, this is kinda necessity.

Dawn Breaker -> Flawless Dawn Breaker - It's a good AOE Ultimate, but... I would Go with Soul Tether on the back bar instead. It gives you a class based Oh Crap! option. You know, when things take a sudden turn to the worse, you can use Soul Tether to instantly heal and stun your foes and get a nice HOT as well. It has saved me many times when I bit more than I could chew. Death Stroke gives you good enough damage as an Ultimate to use against bosses, so having another purely damage based seems redundant to me. Having a survivability Ultimate on the back bar is much more handy.

Well that's all I had to say. I'm sure others have other things to add, and probably wildly differing options about these things.The things is truly that pretty much anything can be made to work. And it is much more important that the build you run feels good and entertaining to you.

One last thing though, if you go with crated sets, and want to keep those light and heavy pieces, then make torso heavy. TI gives more armor than other pieces, so it makes more sense to have that as the Heavy bit. Personally though, I just run on 7 medium since it gives much better passives overall. Especially after the recent armor changes.