It really doesn't matter which faction you pick, because every class basically has a mirror on the opposite faction. Aside from the roleplay aspect of being a "light side" republic hero or a "dark side" imperial villain, there is the very minor population difference on each server which can subtly influence aspects such as guilds and PVP. Basically, pick whichever peaks your interest. There's little to no consequence to choosing either.

Go to your Steam application and click your name in the top right corner, where the minimize button is located. This will show you a drop-down menu - you want to click on "account details". Here you can see your purchase history which includes any subscriptions you have bought through the "manage subscriptions" option. If there is no active subscription for SWTOR, that means it is non-renewing.

Run the game on the lowest possible settings to see if it's an issue with your hardware. Alternatively check your graphics card drivers to see if they are dated or are incompatible. Or, if that fails... you use the repair option on the launcher to fix your game. If the issue still persists you might want to reinstall the game.

There ain't anything wrong with being fat in the SWTOR universe - it just looks ridiculous to see a bulky assassin storming towards his enemies. I mean, look at ya boy Darth Baras. Man's got it going for him.

Downloading the client from Steam and then signing into your SWTOR account will automatically link both together. Subscription bought from the linked Steam account will go on to your SWTOR account - however many people have reported issues with the subscription not working, it is unknown whether it is a Steam or EA issue. As far as I know, the subscription from Steam does not renew itself automatically so after your sub time is over you'd have to manually purchase it again. And yeah, you have to reinstall the entire game if you have had any other SWTOR launcher variant previously.

a mystery for sure, perhaps the archives are incomplete?

Just as your character's stats increase as you level up - so do your companions. Gear has no effect on your companion's stats either.