Actually, fxp is a generic format used by many many VST plugins, you need to know what plugin those presets are for and install that. Then you can load the VST plugin via VeSTige and then load the fxp preset in the plugin.
OR you can just load the .sf2 file by clicking on the pink folder button in SF2 Player so you can use multiple different soundfonts in a project.
That size is kinda hard-coded, you can't easily change it without recompiling from source.
Velocity by standard is a property of a note itself and is determined at the moment of pressing the key down. The thing you would need is called aftertouch. In LMMS you can achieve something similar by using automations.
The question is what exactly you mean by "low quality".
Different issues have different causes, you should describe the problem more specifically.
You normally do this kind of effect by reversing the sample, applying the reverb, then reverse the whole thing back.
It's usually true that the songs you don't take seriously, become the most popular. Because popular music is easy to make, you just have to follow some rules and it's like a template. If you put a lot of effort in a song, what you actually do is customizing it to be perfect for your own taste. But o...
ALSA allows LMMS to take over the soundcard so it can work more smoothly and with low latency. But then you can't play sound from any other program while LMMS is running. PulseAudio is a bit more user-friendly but it tends to mess with your soundcard's settings and introduces some latency. Try both ...
The problem is that they are not in the same folder. On the bottom image you have them in the Program Files (x86) folder, maening they are the 32 bit versions. You have navigate to that folder in VeSTige's browser window.
Also I would strongly suggest getting the 64 bit versions if you can.
It all depends on what exactly you want to achieve, what the final result is going to be. If I understand correctly, the main thing here is the performance itself, so it doesn't really need any more processing in this case.