Good luck with MX, I didn't realise you'd done so much in so little time already, was gonna reply earlier.
1. I tested Chrome on my system, no problems. IMO those transparency issues are a Mint related thing.
2. You should be OK with KDE Plasma on MX. Just bare in mind it isn't their 'flagship desktop' option. What does that mean? It *probably* means that primary/main development happens on the XFCE desktop, ie; more bugs get fixed.
3. You are notorious for running old hardware (I say in a kind way

) XFCE is designed to be less resource hungry, where as KDE is designed to be very eye-candy and hardware intensive. So in my estimation, I'd have gone for XFCE.
4. But that is the beauty of Linux! Choice, you get to pick your poison

Unlike proprietary operating systems. Infact, with a simple command-line option you can install two desktop environments and swap between them on the login prompt. (But this can get messy as each desktop system has it's own set of apps/programs which will be visible to both desktop environments and can get cluttered, so I'd recommend only using one at a time)
Just my thoughts. I just want you to have the best Linux experience possible

I hope MX goes better. The font's being better is already a good sign.