#activating abilities

When you activate Goblin Test Pilot's ability, you choose from among legal targets for its ability during the activating process. If that target happens to be Kopala or anothehr Merfolk controlled by that player, then an additional cost of 2 is imposed on the costs of the ability. You're then free to pay it (and in fact must if you have 2+ mana in your mana pool), but if you don't have the mana in your mana pool, you're not forced to generate it.

Since you then can't pay the costs to activate the ability, the entire activation is undone. You're then free to try activating the Test Pilot again. You're free to do this until the Test Pilot targets a non-Merfolk target if you like, but if you go around abusing this interaction, expect to get some sour looks from the other players in your game.

You're still a legal target for Banefire and will be roasted to a crisp soon enough.

The legality of a spell or ability's targets are only checked twice: The first time is during the casting process, where they must all be legal for the spell to be able to be cast / the ability to be activated. The second time is when the spell or ability is about to resolve.

When you cycle Yidaro, you put its cycling ability on the stack and then its triggered ability triggers and resolves before you draw a card.

No, you can't do this.

Unless an ability of a permanent card in your graveyard specifically says that it can be used in your graveyard, or could only be used from the graveyard, then those abilities can only ever be used while it's on the battlefield.

When you cast a spell or activate an ability, it goes on the stack before any of its costs are paid. Anything that triggers due to the payment of those costs goes on the stack above the original spell or ability and will resolve first.

In the scenario you describe, the soulshift ability of the sacrificed spirit(s) will go on the stack above Lifespinner's ability and resolve before it does, meaning that Iname is still in your library at that point and thus hasn't yet put any spirit cards into your graveyard.

If you have unlimited mana, you can do this, yes. You’ll just have to hold priority and put all the activations on the stack at the same time. If you activate the ability and let it resolve then activate it again, etc., Xantcha’s controller will lose the game before you’ve drawn your whole deck and Xantcha themself will no longer be in play. 

TL;DR: Yes, just hold priority.

Your friend can’t do this, but not for the reason you mentioned.

In order to activate Daretti’s -3 ability, you have to choose a target for it when you put the ability on the stack. You don’t sacrifice anything until the ability resolves and so you can’t target the same thing that you intend to sacrifice since it’s not an artifact card in your graveyard at the time of activation.

If you do sacrifice Spine of Ish Sah as Daretti’s -3 resolves, the return to hand trigger won’t go on the stack until after the ability finishes resolving. 

When you activate an ability, you pay costs for it at the very end, after it has been put on the stack and had targets (if any) chosen for it, etc. Generating mana and paying costs are the last two steps, so you’ll be able to sacrifice Steel Hellkite to Ashnod’s Altar to pay for its own activated ability.