Follow your class questline, profit.

Why don't you just ask your "friend"?

The main purpose of HK-51 is that it's ****** and if you've ever played the original KOTOR should bring back alot of nostalgia

It's per character as I understand it and is instant (apon confirmation).

The legacy redeemable items are designed for your alternative characters, send the item to them through the mail. They can then be traded in at Legacy vendors on Dromund Kass and Coruscant for some better than average starter gear for around the level 10 - 20 bracket.

I recently reached 51 on my main and started the HK questline, so far I've found all of the parts other than the flashpoint drops and the one on hoth. Is it a tedious time consuming task? Certainly. But it is perfectly feasible to do it solo.

In the expanded universe (including the original KOTOR games) there are many lightsaber crystal colour varients. There's nothing really wrong with it and adds to the ability to customize your character.

You won't be able to pick up the Makeb questline until around 47 I believe. If you're interested in the class stories should certainly finish them as Makeb contains spoilers.

The cartel box contents are absolutely random, this is clearly indicated in the description. Are they worth the money? If you're willing to accept the fact that the contents is random, yes. If you get items that you don't paticularly want or need, you can sell them on the GTN or in the case of the rep items (as others have said) vendor them. If you hang on to the items until this pack series is phased out their value will go up dramatically. Currently the rarer items in the Supreme Morgul pack (such as the Pocket Sarlacc) on Tomb of Freedom Nadd go for around 2 - 3mil credits. Then use those credits to buy what you want. Most games require you also to have "keys" for lockboxes, thankfully SWTOR doesn't.

Why not just use the "preview" function?

There are many items in the game that have a high value, it's best to check your servers GTN (auction house) to get an idea of the selling prices for paticular items. Your best bet if you wish to make some quick credits is to purchase cartel boxes off of the in-game cashstore, open them, and hope you get lucky. Several items from the boxes (at least on the Freedom of Nadd server) such as the Pocket Sarlacc are known to sell between 1.8 - 3m credits. With regards to modifications, I'd say don't focus too much on trying to make your gear perfect until you hit the level cap. You're able to get mods, hilts, etc from commendation vendors on planets aswell as quite often as quest rewards, just use those to regularly upgrade your gear until then as focusing on the best current-level mods every time you level up will result in you gimping yourself heavily in the credit department for little gain. Of course in order to use these mods you will need armour that has slots open (e.g Adaptive type armour).

Whoever said it was dead? The population counts seem pretty stable. If the playerbase shrinks they can just merge the existing servers. If the game gets taken offline it's unlikely you'll be able to play the single player content.

There have been alot of issues with collections since the last patch, it could be presumed when they hotfix the issue the items will then appear. It is annoying regardless.

Are you getting any error? After making the purchase click "Unclaimed Items [X]" in the bottom right of the cartel market to claim the items (or refund them back to the market).

It's not really stupid as it does keep the market flowing, just as with everything else that gets bound on use. If they didn't, then the market would quickly get diluted.