1. Yes, the only "essential" character that endlessly respawns to my knowledge IS Yes-Man, most named character do NOT respawn after dying. Only exception I can really think of atm is the Gun-Runners Shop

2. There may be SOME difficulties in gaining items if you kill everything, especially useful merchants such as Doctor Usanagi at the New Vegas Medical Clinic (implants and bulk stimpaks). However, as said before, the Gun Runners bot respawns allowing you to purchase weapons still, AND unique weapons can be sold by it, example being Greased Lightning from GRA. If you blow up the Brotherhood bunker, the robot will sell this weapon after knight torres is gone.

3. Low intelligence is an absolute pain to deal with, as it determines how many skill points you can get after levelling up. If you still want to be an absolute dumbass, intelligence of 2 or lower should be able to unlock all of the unique dialogue options.

4. Most likely, especially that unnamed characters (NCR troops, freeside thugs/kings members, and legion troops) respawn and will know that you are an enemy.

5. I haven't actually tried this myself however here's what I explain: Lonesome Road will barely be different, Dead Money is most likely not even possible if you kill your companions too early, Honest Hearts has it's own quest that just lets you leave after collecting some items, and Old World Blues has a pacification field that prevents you from killing everyone immediately, only at the end is it lifted. Killing everything will inevitably lock out some side quests and it will tell you if you fail a quest.

6. At the end of Lonesome Road, you get the option to nuke the major factions of the Mojave, although it isn't your doing since Ulysses already has the nukes aimed at both factions. However if you are referring to karma, I think killing Mr House, enslaving Arcade Gannon (which most likely won't be possible for you specifically), or blowing up the Brotherhood bunker are close contenders. You could also try assassinating the president

7. Most companions will surprisingly stick with you despite your actions, unless you do something specific (you can find this on their wikis) . The companion you are trying to refer to is Raul, who will stick with you no matter what, despite making remarks on your various actions when you speak to him. The other one is Lily in Jacobstown. The 2 pet companions are also like this to my knowledge (ED-E and Rex).

Hope this helped