Just my 2cents as 2200 QM MMR and about the same HL. 1 & 2) As Uther after you get accustomed to the standard build you may want to look at your team and the enemy team and decide if you will be front lining Uther or ranged healing Uther or baiting Uther. You may need to go for cool down stun if your team doesn't have much cc. Sometimes it may be good to go divine storm if the team has a lot of melee assassins and warriors. Or you may want to go divine shield with a Butcher or Nazeebo or Illidan and etc. Sometimes you need to be bait Uther and go spell shield and imposing presence if your team is very aggressive. At least that is how I play it whether correct or not.
Q3. Alot of people go with talents that have synergy together and keep you alive. Dead hero does no damage. I think a lot of the builds focus on that. Shields give you health and mana regen if you spec it to get back in the game. Eviscerate lets you either attack from a distance or run away to a distance if there is a target there. When with the tank you want to protect the tank, let him do the range damage. Eviserate will help you get to the front but also possibly put you in danger. With shields you can protect the tank and main focus would be to stun anyone that attacks the tank in my opinion.
Q4. I think at higher MMR people watch the talent usage of their allies and enemies more so their is more combos and timing rather then when it is immediately available. Lili timing blind for Butcher. Muradin timing his toss and stun to follow up Kerrigan or to disrupt Jaina combo or something.
Q5. Sometimes people aren't on their main so I don't necessary think I am better. Just that their play with that hero is costing us this win but usually there is someone else on the enemy's team that is bad. For example my team had a hammer and kael but the enemy team had a less noticeable soloing Uther or Tyrande that was often out of position later in the game.
Q6. Generally I don't worry about comp in QM and some HL since at my rank you can pretty much win with almost anything.. but sometimes you can be completely screwed. That is not to say pick whatever but more of as long as synergy exist. You can take out warriors with ranged assassins. A team of valla raynor and tychus would push out a lot of damage. Maybe you would put Kerrigan and Thrall for some stuns. The tanks would need alot of hero that can initiate and disrupt positioning I would assume so diablo, stitches, muradin, etc, anubarek. So it depends what you get not considering leveling and wave clear. Maybe 33% you will win, 33% you will lose and 33% it will be close.