It is possible to play a dk dps and healer. You just have to practice at it. As far as tanking goes I believe it is possible but will require the right gear and resistances. The only problem I could see would be not having enough stamina

I tank the daily pledges with a magicka DK and have very little stamina or stamina regen. I have about 32k magicka and 2100 spell damage. I use sword and board on both bars and the blue health/magicka regen food from Wrothgar. I don't think a resto staff would add anything. If you have ten mobs on you then you're not going to be channelling heavy attacks and all your healing is covered by DK abilities.

You only need tons of stamina if you're trying to taunt lots of trash mobs and block everything. I just get in there with Eruption, Talons, Deep Breath, Flappy Wings, Spiky Armour and Igneous Shield. I very rarely have to use Coagulating Blood as Deep Breath covers all my healing. Most packs of trash die so quickly these days that you just need someone to absorb the alpha and get everything's attention.

For bosses you just use heavy attacks to keep Puncture going and block the big attacks. Otherwise I'm keeping my buffs and dots up.

You can easily switch to a healer or dps by changing gear. I use an Outfitter addon to quickly switch roles so I can do all three. You don't really need a tank for silver pledges though. Just go with 3 dps and a healer, or 4 dps that can look after themselves.

My main is a tank and a dps. I carry two separate armor sets and about 10 different weapons for varying situations as well as multiple types of food and drink. I can be ready to go for either role in less than 10 seconds with quick swap gear and skill add-ons. I am incredibly successful. It is very possible as long as you know your builds and balance your stats to be enough health for tanking and plenty of stats for dps. I put twenty points into health and the rest into stamina and I balance the rest with enchants, food and drink between the two sets.

You can be both on one character, but I suggest you not take more than one role in a dungeon and just go with either dps or tank and stick with it for the area. It's going to be a bit of a bother changing gear depending on role, so I would suggest you get some sort of addon.

You can absolutely run a Magicka Tank without swapping gear! I've created a DK Dunmer Magicka Tank that has a very offensive bar and a very tanky bar. Here's my setup:

Valkyn Skoria- Hat (divines) and Pauldron (divines)
5 PC Pariah- (3 Jewelry <shield play enchant, reduce spell cost x2>, divine gauntlets and sabatons)
3 PC Kagrenac- (1 divine light belt, chest and greaves heavy, divines)
2 PC Kagrenac- dual wield 2 nirnhoned swords on the offensive bar and 1H/S on the tank bar (defending sword, divines shield)

The extra resistances from Pariah allow me to put ALL of my attribute points into Magicka. My offensive bar also allows me to have a ton of health and magicka from the ability's passives.

Offense bar: Burning Embers, Molten Whip, Mage Light, Structured Entropy, Eruption Ult: Meteor
Tanking bar: Green Dragon Blood, Pierce Armor, Volitile Armor, Heroic Slash, Obsidian Shield Ult: Corrosive Armor

The build works because of this:

Valkyn gives you health and extra DPS (that you don't have to put effort into)- it synergizes perfectly with a DKs World of Flame and other passives (especially if you're a dunmer)

Kagrenac gives you health, max mag, regen, AND spell power. (I couldn't decide between TBS and Kag- but ended up figuring out that Kag gives more spell power when stacked with 8 divines armor traits... if your mundus is The Apprentice)

Pariah helps get you to max resistances and offers a "mitigation" of sorts in its 5th trait. I'm also a vampire, which offers more mitigation at low health. It seems that they stack well!

Dual wielding nirnhoned swords increases your ability's spell damage and spell pentration. The fact that you're all divines, maxed out in magicka, and using two light armor pieces makes your Burning Embers, Molten Whip, and Meteor hit HARD. Structured Entropy, Mage Light, and Meteor also add regen, reduced cost, Empower, and Might of the Guild.

This is my latest (and fourth) V16 character and I enjoy this toon more than my three Nightblades... let that sink in.

You will never be able to do full on dps numbers if you try and do two roles at the same time, that being said its fully possible for you to allocate your attribute in such a way that simply changing armor will change you from one role to the other, something that is necessary for vWGT and highly appreciated in vDC. As long as your tanking responsiblites don't suffer from it I would still encourage you to do what ever damage you can, but that should be after you make sure to keep the taunts on targets that would require it. For example not taunting the big 2h guys in vCoA and letting them loose on the pure DPS's will hinder their ability to perform to the max, lowering the overall DPS of the group significant and making it take longer or downright killing someone. Be solid in one thing before you try to do two things.