1.27 has more advantages than just the autosplitter btw. So the answer to your 2nd question is only a relative yes, there is more to it. Viper made a good video about it:

You don't need a timer on screen to submit, we retime every run anyway.

LRT basically just means removing the time you'd spend waiting for the game to load. We do NOT time any runs using LRT, so you should leave that empty. We just cannot remove the option for LRT because of src.

RTA is just the default timer you probably used anyway, so fill that in.

enable autosaves, play through prologue and keep that savefile.

If you are on PC you can always download a savefile (integrated in P1.27 if you have that or just download it from the resources section here on src [any% classic no om0 -> trevor% -> f/l])

I think you need to just download the file on their website and put the dll in your livesplit/components. It should be documented on their website, worst case you can also ask about that on the discord

There is nothing like a well documented stratbook or sth, best we have is the tutorial series in Guides. Trevor% is a little older, but still for the most part up to date, especially regarding beginner-friendly strats. That aside you can join the discord and 1) check the confirmed strats channel or also 2) just ask around, especially if you decide to watch WR and don't understand some stuff.

People usually don't share their notes, mostly because (from my experience) notes are very personalized, some people may even skip over some strats in their notes that you'd find essential.

Also, writing notes is of huge help to memorize the stuff yourself, so I HIGHLY recommend writing them yourself regardless.

Also if you're on PC, Livesplit has a sweet extension called SGL you can use.