I just finished 2 tonight, and I'll say I DO think it's worth playing 1 all the way through first. Sorry to hear about the spoilers discouraging you, but if you absolutely can't get back into it, 2 is still a masterpiece in its own right.

This thread is a little old and I'm sure everyone has given you some good advice, but I'll see what I can add. Big fan of the games but it's been a while since I played XC1.

• Your party should usually be set up like an MMO. You'll have a healer, tank and DPS. Shulk is your DPS as a lot of his abilities attack from the side or back. What you want to do is let Reyn take the damage and run around as Shulk and attack enemies. You'll get a healer before too long (the chick with the rifle). That party will take you through the main game. 
• You can choose any character, but I would stay with Shulk most of the game. He only gets a few arts, so try and learn them. Basically, move into position and use them. 
• Notice that gray circle in the middle of your arts? That's the character's unique ability. For Shulk, it's the Manado arts. Early on in the game, you should just make sure it's full and only use it to give yourself the ability to attack mechs. Once you get past Colony 6 (first major area), you can experiment more. 
• When you get to a town, try and pick up all the quest you find. They are your best source of money and experience. If you are obsessive and do all the quest you can, you'll be massively over leveled. You also don't have to turn a quest in, so if you get all the items or kill the enemies, the quest automatically completes. There are some you have to go back to town for, but these are the important quest which usually gives you more money and experiences. You don't have to worry too much about the quest, but at least do them as you explore will give you the boost you need without mindless grinding.
• You will probably see a little blue gauge in the top left corner. That's the party gauge. It fills up when you get good hits in (like using Back Stab behind the opponent). You'll see a little symbol around the damage numbers when you did it right. This fills up the party gauge. Shulk is very good at doing this as all of his arts based on position will increase this. One bar can be used to revive a fallen ally and if you die, your party will revive you. You can also use all three for a chain attack which can do big damage. You should only do a chain attack when you are sure that you won't all die after doing it. I don't remember how it exactly works in XC1, but once you understand it, you can do big damage. I think it's based on everyone using the same color.
• There is a thing called topple in the game. If Shulk uses an art that "Breaks" another character can use an art that "Topples." The enemy has to have Break first. They will be incapacitated for some time. You can then do daze, which Shulk has though I don't remember the benefits to Daze. No character has the full chain (i.e. Shulk can't topple but can break, Reyn can't break but can topple). If you set it up, your AI allies will follow up. Another good reason to stick with Shulk.
• Remember you can upgrade arts. These also decrease the cooldown time
• Xenoblade 1 did this weird thing where enemies at higher levels are harder to hit. Just be mindful if you're attacking enemies higher than you are.
• Don't let the AI control Shulk (as well as the mage character you'll get later on). It has no idea how to use them effectively. The CPU also struggles a bit with your healer, but is serviceable. The rest of the cast it can do alright.

The High Entia tomb was pretty cool. I liked that story arc with Melia.

Glad you're able to still enjoy it. Without validating or denying that spoiler, you're in for quite a ride. While I enjoyed the story at that point, the area you reach after Alcomoth is where the story really picks up.

If that's what was spoiled, then don't fret. You're at the point of the game where the story starts to ramp up. Let's just say there is a lot more you don't know. 

As an aside, you mentioned Melia. If you are interested in using her, she's really good but you need to control her and I would look up how she works. IIRC, you use arts to create orbs which give you buffs and then use the center ability to fire them off for damage and debuffs. I didn't get it at first and if played properly she does a ton of damage. 

That's a fine team too. My first playthrough was something like that (though I subbed Shulk for Riki) You can always sub Dunban for Reyn if you need the hit points since they play the same roll. Seems like you have the right set up. You'll be good!

Melia has a +15 Agility skill that takes very little points to learn, link it to everybody. Enlightenment (last skill in the same tree) is important to be able to hit the post-game enemies with ether attacks (just in case you want to try fighting them). Sharla likes Melia's skills that boost ether.

Riki has really good skills to link. He has skills that gives the same Exp and AP to characters who aren't fighting. He's the only character with skills that increase the chance of getting gold and silver chests, which makes getting crystals and gear way less annoying. He's the only one with cooldown-reducing skills too (those are really good as well).

I played with Riki and Melia a good amount of time to have them get more skill points, and they were able to learn all their skills minus one.

All the quests in Alcamoth are timed. You have a good amount of time to do them, though. After you've finished with Prison Island the quest to unlock the 4th skill tree for one character will be available somewhere in the Eryth Sea. You'll have to do a few others first in the same area to unlock it, though.

Ways to get affinity quicker?

1 - Accept and complete quests. When you accept or complete a quest you'll see a bunch of pink hearts during the dialogue. It's ok if you don't want to bother doing them because there's no penalty for picking up a quest and not completing it.

2 - Heart to hearts (or whatever its official name is). Those dialogues in specific locations where you have to choose between two options 2-3 times. Very limited.

3 - Item gifting. You can have one character give another one an item. This is restricted to items you pick up on the field. Be careful because you may need the item for something else later (I guess you're safe if you have many copies of the item you're going to gift). Quite slow. Characters may like or hate the item given to them. As a rule of thumb Melia likes flowers, Sharla fruits and Shulk mechanical parts.

You can't find Shulk's quest giver because you need a ton of area affinity for that quest to be available.

If I remember correctly the game will start going crazy at that point. I won't tell you how but keep your expectations low

I remember playing this area on a Sunday afternoon/evening for five hours straight. Afterwards I had a really bad night since I dreamed the whole time of the radiant crystals. I would love this area if I would not need to find any ice cabbages  

Rainbow Slugs... not a single one in 200 hours of play. They're the reason why I don't have a 100% collectopedia completion.