The compleated ability looks only at whether a player chose to pay 2 life for a Phyrexian mana symbol as they were casting the spell. If a player paid life for some other reason while casting the spell, that will not reduce the number of loyalty counters the planeswalker enters the battlefield with.

The last loyalty ability will check Nahiri's loyalty as the ability tries to resolve to see if the target is still legal. If it's not, the ability won't resolve and none of its effects will happen. The card won't be exiled and no token will be created.

A hybrid Phyrexian mana symbol contributes 1 toward the mana value of a card, even if life is paid for it. Specifically, Nahiri's mana value is always 4.

Other replacement effects that would change the number of loyalty counters Nahiri enters with will apply as normal.

If the creature targeted by the first loyalty ability can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost. If they don't, it doesn't have to attack in that case either.