Lex and several others claim it works fine on Eclipse and while it does appear to be broken in intellij, intellij has a decompiler built-in so I'm going to save my sanity and stop looking at the horror that is this codebase.
I'm honestly not that knowledgeable in the way of performance, but I believe the team don't tend to take poor performance claims seriously without information from a proper JVM profiler and not Minecraft's F3 menu
For most mods the point is to add content and that tends to require the client to be aware of the content that is being added.
No, you can't simply "edit the jar", that is not how any of this works.
No, there is no way to do this from the user end.
I will always recommend using Twitch to install packs but not run them, the "Logs" section of my signature, "Twitch" subsection contains a video on how to run Twitch modpacks with the regular Minecraft Launcher.
I say this because "Failed to find Minecraft resource version" is a common problem with Twitch managing to screw up Forge's install process