First question: is this your first/primary character, or do you have others on your account? Will there be others? I ask because this will greatly affect how you go about earning Champion Points.

Now, as to your idea. I think it can work great. I think you could conceivably level to 50 and earn CPs well under such a character. It might take a little planning. Some tips:

Class and race: I think this character has to be a nightblade. Cloak is just too important for your role play idea to accept the other classes' invisibility mechanisms. A magblade or a stamblade would work, although I find stamblades to feel more like assassins and magblades to feel like dark sorcerers. But either works. Khajiit, wood elves and red guards make good stamblades. Altmer, bretons or dunmer work for magic. But you can mix up race since you aren't wanting to max the character.

Skill points: by giving up the faction quests, you're foregoing 75 skill points. However, with One Tamriel this is less of an issue. Because you can go anywhere, there are plenty of shard skill points. When I roll a new toon now, I don't quest. I get the shard points, and usually this is enough to have them fill out their fighting trees plus the basics in crafting. (They can spend 10 points in, say, smith to make rubedite and do highest level writs. But it'd be tough to be a master crafter and spend on all the passives without quest skill points). It will also really help if you have access to all the DLC shards.

Leveling to 50: this could go as fast as everyone else or very slow depending on how you approach it. If your toon goes on murderous delve rampages (and what assassin wouldn't?), killing everything and clearing the boss...then maybe also does dolmens and later undaunted, you can level to 50 as fast or faster than questing. If you cloak through delves, then yeah, it's gonna be a slow grind.

CPs: the same rule applies. There are plenty of ways to earn XP, and honestly quests aren't the most efficient. If your assassin kills a lot of things while she explores, then you'll earn CPs just fine.

Routes: the play style you're describing isn't far off how many vet players approach a new character, especially a nightblade. Having run the quests three or four times, there's NO WAY I'm playing them again on character #11...ugh. So I run them on the sort of dailies you mention. For a while on my newest nightblade I'd do writs, then run the dark Brotherhood daily, the kvatch world boss daily, a thieves guild repeatable then the thieves guild heist. I can push out all of this in under an hour, which is a lot of xp. Routes like this will make it possible to max out your CPs...then you might want to supplement with some psijic dungeon runs every once in a while.

Frankly, if you aren't stressed about getting to vet or getting to 630 champion points, then I think your character idea will be a lot LESS frustrating than people who demand best in slot max gear. Those guys end up running the same dungeon 135 times to earn their sharpened dagger of who-gives-a-flip. When you aren't doing that you can spend a lot more enjoyable time just wandering and playing how you like.