First of all, you don't need Trinity if you have Entrenchment and vanilla. All you will get new is Diplomacy, which you can get for $10 on Impulse, compared to the $40 of Trinity.
1. Most MP games use quick start, but their are a handful of people that play Entrenchment and still don't use it
2. Well, if you have a well balanced force to defend it with, a big LC fleet can do some nasty things, particularly with the Advent, since it can have 3 SC instead of 2. I think SC always were important, and they are even more so in Entrenchment. Well, remember that flak never was really THAT good against anything except Advent and small groups of TEC fighters; Vas bombers can stand up forever against most flak groups, so in summary, flak is only a little, to stick with your metaphor, a little over par.
3. I never was aware of a 3rd micro-expansion
4.https://forums.sinsofasolarempire.com/348928; this could give you some advice on other's opinions, because i think that your question is kinda an opinion choice.
5. A well balanced fleet is a great choice, expecially an Advent one, but a TEC one is good too and the Vasari have a tech that increases fleet capacity by 8% to 15%, which makes up for the Vasari's higher fleet logistic requirements and then some. A LRM spam can be easily countered by a LC spam w/ fighters, or the Illums (but illum spam can beat just about anything, except a bigger Illum spam).
6. I don't know about hard AI cheating, i think that the Unfair and Cruel AIs are the only ones that can cheat, but i didn't do too much SP when I got Entrenchment, I did about 5 games per faction, one against each AI type, 2 for unfair, to get used to the new stuff, then I went straight to MP.
Hope this helps.