Forge includes some HD patches. Most HD texture packs will work.
If not, just uninstall Optifine until the maker fixes it, and go back to vanilla pack.
Tekkit uses a build of bukkit with integrated forge - but it is not an official forge build nor an official bukkit build hence why neither can help you. Failing Tekkit forums, try MCPortCentral forums.
What would tell the server which mods it has joined with? The client.
You see the issue there? A client could just lie about which mods it has installed - and you'd be back to square one.
No, it is on the forge files website - branches are at the bottom. Have you tried without MultiMC? This isn't fully tested yet, so it is no surprise there will be issues.
I have actually seen this issue before recently. The problem is the quotation marks around %s in fml.py - on some systems it doubles them. No idea why, but removing the quotation marks should work.
While that may be true, Modloader is less picky when it comes to those things - so perhaps FML should sort it out for Modloader mods which try to do that (if possible). Are you just using Modloader functions?