My question to you is do you want something that's free or do you want to throw some money at it? If the former, there are quite a lot of good soundfonts out there if you do some digging (edit: actually I might have some on my pc, could share later). Otherwise it depends on your budget and what...
X sounds more harsh in the highs to me but it also sounds more glued, it's not a huge difference, hard to pick one over the other in this case. Was there no 2bus processing (not even compression) on the "unmastered" track?
Also there's no way x50 or gtune are causing 15 samples of latency, it's something else, possibly your cab sim (I know recabinet introduces some minuscule amount of latency). Still your daw should compensate for this, you didn't mention which daw you're using.
Don't think we'll see RTAS from any freeware dev soon. I hear Avid are really picky with who they give out licenses to for their SDK (basically you have to be running a business).
It's a misconception that IRs "model" the power amp as well, since IRs can't recreate non linear behaviour (power amp tubes), you'll only get a snapshot of the equalization. But you're right, power amp simulation isn't required to get a good tone.
Sure the guitars are sloppy and out of tune and cloudy, but what's bugging me the most is the almost inaudible vocals. If the guitars suck then gut 'em and make more room for other instruments and especially the vocals, but perhaps it's too late to make such a drastic change.
I suppose it depends on the aesthetic you're going for and how busy the arrangement is. Some material sounds great with a bit of low mid warmth in the guitars and bass guitar, other material might sound better with a more scooped sound. A mix doesn't have to look like white noise in an analyzer.