You're not mining unless you bought special hardware for it. Core and Knots don't even give you the option to mine anymore because it's so inefficient with a regular CPU/GPU.
It's hard to say. If 80% of nodes were Knots, it would be harder for data transactions to propagate. But if there's enough market demand for those transactions, wallets could prioritize sending them to the 20% of non-Knots nodes, so they'd still easily get to a miner.
As far as submitting directly to a miner, the only pool I know that offers the service is https://slipstream.mara.com/ and yes, it is currently about 2x as costly as current mempool fee rates.