If you sacrifice Party Thrasher while casting a noncreature spell from exile (to activate a mana ability, for example), the spell won't have convoke when you pay its costs unless it has convoke some other way.

Multiple instances of convoke on a single spell are redundant.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

When using convoke to cast a spell with {X} in its cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell's total cost. Then you can tap creatures you control to help pay that cost. For example, if you cast Wrath of the Skies (a spell with mana cost {X}{W}{W}) from exile, choosing 3 as the value of X, the total cost is {3}{W}{W}. If you tap a white creature and a red creature, you'll have to pay {2}{W}.

You pay all costs and follow all normal timing rules for cards played using Party Thrasher's second ability. For example, if the chosen exiled card is a land card, you may play it only during your main phase while the stack is empty.