2) This is a local-only change that does not require consensus. It's okay for light nodes to still follow the most-work chain. What this change does is provide miners specifically with a higher chance of ending up on the final chain, by better estimating which fork has the most hash power behind it.
Not sure why you call it "local-only change", because as we are discussing here, it appears that there's a risk for netsplits that don't re-converge, especially if different nodes will follow different rules.
There is no possibility for non-convergence. The most-work tree eventually wins out. Always.