Does Lifelink only apply to combat damage?

Rulings. The ability causes life gain whenever a source with lifelink deals any damage, not just combat damage.

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Can Mirage Mirror Target graveyard?

The moment Mirage Mirror hits the graveyard it stops being a copy of whatever you copied, but "dies" triggers use the last-known-information from the battlefield to determine if and how they trigger. And another question, can mirage mirror copy graveyard? Mirage Mirror will wind up as a copy of the permanent targeted by the last ability to resolve. If Mirage Mirror becomes a copy of an Aura, it's put into its owner's graveyard unless it's somehow attached to an appropriate object or player already.

Keeping this in consideration, does becoming a copy trigger etb?

You are correct that a clone copying a creature with an ETB like shriekmaw will trigger the ETB. Clone and its freinds go seemless from a clone spell on the stack to copy of whatever you chose to copy as it resolves and enters the battlefield so the game sees the chosen creature ETB. Keeping this in consideration, can i copy a creature spell? Yes. You can copy them. In addition, since you are copying a spell on the stack, you can copy things that a creature-copy normally can't. Like the value chosen for X (so your Lightning Serpent token gets counters) and kicker (so your Desolation Giant token won't have to destroy itself).

Keeping this in consideration, is clever impersonator a creature?

Clever Impersonator is not a token. In that case, Clever Impersonator enters the battlefield as a 0/0 creature, and it's put into the graveyard immediately (unless something is raising its toughness above 0). Can clever impersonator copy an enchantment? Yes. See the rulings quoted above. and finally, what about if I copy an enchantment creature that has been bestowed onto another creature? It doesn't copy the effect that makes it an Aura, so it enters in the regular Creature Enchantment form, not bestowed.