From what I can gather, it doesn't seem like there is a way to know your "inputs" upfront. The inputs are the various addresses the different chunks of bitcoin are coming from... I think? So the inputs, as I understand, will depend on the source.
Your wallet's balance is comprised of the various 'unspent transaction outputs' (UTXOs) it controls. These UTXOs ...
No, in general non-miner third parties cannot influence the priority of transactions except by outbidding them.
Unconfirmed transactions can only be reprioritized by either the sender or one of the recipients:
Any recipient (including the sender if there is a change output) can craft a child-pays-for-parent transaction that will incentivize the inclusion ...
The signature signs the spending transaction(*). Modifying it in any way will invalidate the signatures in it.
(*) Not actually, as it's obviously impossible for the signature to sign itself. There are also means for a signature to indicate it explicitly does not sign certain parts of the transaction, permitting those to be modified after the fact.