The combine VST and VST3 plugins in FL Studio option will combine the VST versions to make the list look shorter, and FL Studio will try to use VST3 where possible..
This is actually a really good question, and I found this forum post where reflex described the process of how the VST combining works pretty well.. it’s based of the VST2 ID or something, as sometimes you can still have two plugin versions in the list with the option enabled (from my readings there)…
Regarding a shortcut to open the Plugin Manager, I don’t think there is. You actually shared a faster way of accessing it with Options -> Manage Plugins..
I would usually just go F10 -> File -> Manage Plugins
I don’t think at the moment there’s a way to create a shortcut to the Plugin Manager, as a lot of these shortcuts I’ve created off the M-Audio Oxygen Pro Drum Pads are based on MIDI Scripting, and what is available right now.
Also, here’s articles I’ve written on the Plugin Manager.
HEADS UP – When combining VST2 and VST3 versions, you may experience the odd bug, such as things like maybe sidechain inputs not being available the same way across them.. but when combining 32-Bit and 64-Bit plugins in FL Studio, this feature should work well without any issues…