Still don't understand everything but I will read more.
If all miners were working on p2p pool could payment processors leverage the shared shared chain for instant transactions with lower chance of double spend?
I don't believe so. P2Pool doesn't synchronise individual transactions between peers; and only the coinbase has restrictions. This means peers organise their own transactions (from their own node).
Remember that chance-of-doublespend is probabilistic with time, not blocks, so simply producing more blocks doesn't provide more security, and if orphans can't be integrated into the chain then that PoW is wasted. This means that lower block time (and hence higher orphan rate) may actually decrease security of the chain if the orphan rate is significant enough.
Bitcoin Wiki P2Pool pageGHOST explanation from Ethereum Whitepaper