1 - I am unable to press multiple buttons in a game at the same time. So for example in Yie Ar Kung-Fu I can't jump in a direction, only up. Same issues occur in other games.
2 - I got a JoySnes adapter to be able to use my SNES controllers and everything seems to work except for the up bottun on the joypad.
You can try to install bios and roms of AX350 or even AX500 then check the problem. You will need to remove the IC to reprogram it though. I can install AX370 files on my Yamaha YIS-503II if you want and report back within few days. But I am not sure if that will help!
This doesn't sound right. AX370 can handle two button combinations fine (e.g. up+space). It struggles with three button combos though. For example, pressing up+right+space will only register up+right but not the space bar press. I find this pretty annoying in shooter games like Knightmare and Salamander. Joysticks connected to the AX370 behave properly and have no such limitations, and of course they move fine in all directions.
I feel in your case you have a problem in that particular machine. The AX370 in general doesn't have the problems you're facing. Maybe deep clean the keyboard? You'll be surprised with how much garbage is hidden in there.
As for the SNES controller, does the same happen in joystick port 2? Did you try the SNES controller on a real SNES to make sure it's not faulty? And if you have a Sega joystick, you can try it on the MSX to see if the directions work fine. This way you can identify which part is the problem.
