First is more feasible than the second. Just including the second for kicks, and because it makes {some} sense "reality wise".

1a. Have races with an elemental vuln no longer become resistant to that element when affected by "resist element". Instead have "resist element" make then take damage like a "normal" person would who is neither vuln nore resistant.

1b. For races with elemental vulns, let "resist element" stack if received from different sources. Possibly have it be stackable only if one of the applications is from worn gear (e.g. ancient hide).

2. Have stoneskin nullify metal vulns for elf/drow kind.