Newbie questions about the Witch

To put my questions into context, you need a little background. I am an older player and my health is horrendous. I have suffered TWO strokes, and have arthritis. The strokes affected roughly a third of my brain. Unfortunately they mostly affected my motor skills with a bit in the cognition area. While relatively smart, I no longer pick up on things as quickly as I used to, and I definitely have reaction-time issues. I usually counter these by playing pet based classes, such as WoW's hunter and warlock. FFXIV's summoner wasn't quite the same since there was no tank pet. So on to the questions:

1) How easy will it be for me to absorb the mechanics of this game?

2) I plan to go necromantic witch. Are the minions relatively smart or merely attack whatever I direct them at?

3) Do the various minions have roles, such as tank, melee dps, ranged dps, aoe dps, or even mild heals?

4) In other games, I often die because I don't see projectiles or ground effects in time for my limited reaction time to adjust. How forgiving is this game?

5) [General] I am story-driven and don't care too much about typical end-game activities such as ladders or ranked content. I understand I only have to do the campaign once, but can I elect to do it again on harder difficulties if I choose to?

Thank you for your time.

i'll try to boil it down to it's essence.

1) there will be a way to make your minions "aggressive" or "defensive" the simplest way to do it is;
the support gem "Feeding Frenzy" for aggressive
the support gem "Meatshield" for defensive

2)Aggressive minions target mobs from pretty far away and do their best to run around causing havoc on your screen.
Defensive minions target the enemies closest to you.
Minions also prioritize the cursor as they are summoned, because they'll try to move to that location. if you use a minion skill (skeleton, zombie, but also convocation and offerings) all of your permanent minions (zombies, golems, etc) will target that location first.

2) you can tweak any minion to do whatever role you want them to fill.

That said, Zombies and golems are easier to build for health/ defense

skeletons and Raging Spirits are easier to build for damage

Spectres depend on the corpse you summon them from, but can fill a big utility role.

Guardian depends on his gear, so giving him gear that grants buffs to allies or curse on hit gear is recommended. note that if the guardian dies you'd have to equip new gear on him.

4) don't play on hardcore mode. screen clutter can mask some instant kill effects.
you lose 5% exp on death, you wont notice this at first, but later on (level 90+) this number (while staying the same) can feel really punishing.
if movability isn't your thing due to medical reasons i suggest you build a tanky character. keep your resistances topped up and get some life and armour and eventually top it off with 100% spell suppression.

5) you can revisit areas you have completed, but they will be at the same difficulty. Every time you make a new character however, you still have to go through the campaign first before you hit the endgame portion.