2) Even better: daven in a different minyan where you will be able to start at exactly the same time as everyone else.
To daven with the chazzan is tefilla b’tzibbur according to SOME opinions.
To start when they are still in the 1st bracha, or maybe even the 1st 3 brachas, is probably l’chatchila, though it remains preferable and ideal to start at EXACTLY the same time as everyone else with no delay whatsoever.
Personally, I do not like to daven with the chazzan because I find it distracting to have to keep up, and I like to daven when it is quiet in the room/ beis medrash.
3) Yes. Or one who is too weak to stand.
4) Some poskim hold that way. I believe the Arizal davened this way.
If you did daven this way, you would certainly be yotzi b’dieved.
6) As mentioned, it is the custom of bnei Torah to stand for the entire duration of chazaras hashatz.
8) There is no prohibition to repeat words within a blessing if you are not sure where you are holding and you want to repeat words to make sure that you said all of them. In Shema, every word must be said to be yotzi so in a case of doubt obviously you would go back because you need to be yotzi krias Shema.
9) You would answer the main parts of Kedusha (Kadosh, Baruch, and possibly Yimloch) if you are holding by Elokai Netzor.
Also, if you just finished the 2nd bracha of SE, and the chazzan reached Kedusha, then you would say Kedusha.
10) OK to look into a sefer while waiting for chazzan to finish Yishtabach (and you already finished it).
During leining – in-between the aliyos Rav Moshe zatzal would learn Mishnayos.