Resolve stuck transactions as 3rd party and without the need for reciever's private key

Resolve stuck transactions as 3rd party and without the need for reciever's private key

As far as I know there are few ways to resolve an unstuck transaction.

1. CPFP (child-pays-for-parent)
2. RBF (replace-by-fee)
2.1 FSS-RBF (first-seen-safe)
2.2 Full-RBF
2.3 Opt-in RBF

My question is simple:

Regardless of the wallet type, If USER A does a stuck transaction to USER B. How can USER C (myself) resolve unstuck transaction without the need for a private key?

Miners charge X amount to push unstuck transaction to their next block.

Opt-in RBF, is possible only if user opted in to replace in advance.

I find FSS RBF quite expensive for cases with big transactions. If the transaction was originally 3 bitcoin then to create RBF with higher fee would require again 3 bitcoin which is quite expensive.

FULL RBF is not supported by many wallets.

http://ift.tt/2A6B7th