I've written a couple of blog posts on replace by fee and double spending mitigations. They sum up the last few years (!) worth of discussions on this list and elsewhere, from my own perspective.

I make no claim to be comprehensive or unbiased but I keep being asked about these topics so figured I'd just write up my thoughts once so I can send links instead of answers :) And then so can anyone who happens to agree.

(1) Replace by fee scorched earth, a counter argument:

https://medium.com/@octskyward/replace-by-fee-43edd9a1dd6d

This article lays out the case against RBF-SE and argues it is harmful to Bitcoin.

(2) Double spending and how to make it harder:

https://medium.com/@octskyward/double-spending-in-bitcoin-be0f1d1e8008

This article summarises a couple of double spending incidents against merchants and then discusses the following techniques:
  1. Risk analysis of transactions
  2. Payment channels
  3. Countersigning by a trusted third party
  4. Remote attestation
  5. ID verification
  6. Waiting for confirmations
  7. Punishment of double spending blocks
I hope the material is useful / interesting.