Also fast question, can the keyboard key-caps be removed?
Mine are a bit dirty...
In short: no. They were not designed to be removed (contrary to some very similar models such as the Yamaha computers).
Long answer: If you just try to lift them, there are some plastic parts that will break. But they can be removed with some tools and patience. I needed to do that since on my AX-350II, some keys were not working at all since thay had some sand etc. inside them! But, the whole process is so tedious that I've never come around to it (I "refurbished" just the most important keys, but all of the keys are in a need of a similar overhaul).
In case you break some keys, they can be fixed with some thread and superglue; but this is fiddly and will require some patience.
I have some pictures ... somewhere. I'll try to find them and post them here if I do. In the meantime, you have been warned 
Ok, here are some pictures.
Disassembling the keyboard is somewhat straightforward (remove screws etc), except for the keys with LEDs. Those need to be de-soldered before the keyboard can be disassembled properly (IIRC this part is similar to other Yamaha computers with similar keyboard and described in their SM). No pictures of this part, sorry (IIRC I might have taken them but it was nearly a year ago, and if I did, they are lost somewhere).
EDIT: The keycaps come off the plate they are attached as in the CX5M SM linked previously. However, the actual mechanisms are different (probably cheaper to construct?), and only CAPS and CODE caps are removable just by lifting them off as in the SM. As for the others, see below...
The keyboard is a membrane keyboard:
With springs housed inside the assembly the actual keycap is attached to (this is just the "top" part!):
The whole keycap assembly is quite easy to remove from the plate; however getting to the part with the spring (which will also collect dust, sand etc. and make the mechanism not work properly) is not easy, as the other half of the assembly will get easily broken. Here is a broken and an intact problematic part:
The easily broken part is under the keycap and nearly impossible to get to. If you just lift the keycap, it will break! However there are small holes at the bottom of the assembly that can be used to bend the flaps away so that the assembly does not break. Here I am trying to remove the "halves" from each other with the help of partially straightened paper staples (result: Success!):
Here I am using some very strong thread (bear thread) to wind around the broken parts. Aftgerwards I soaked the thread in cyanoacrylate -based superglue. (I used more laps than in the pictures). Result: The keycap does stays inplace now! The fixed keycaps are indistinguishable in use from the originals, btw.
There are more pictures in the album (from different angles etc.)!
Incredible how-to Wild_Penguin! Done like a boss, Thanks a lot for all the info 
Also I have finished cleaning the keyboard module.
Maxis have mentioned that the AX-350II has the YIS-805 circuit with few modifications, dose anyone have the schematic or service manual for the YIS-805, so far reading the CX5MII showed clear differences in board layout and system upgrade-ability
For example the bord seems to have empty unpopulated spaces for extra ram perhaps, but in the manual it show no hint of 256k model or upgrade...
Just checked by old docs. In order to get 256kb on a 128kb YIS805 (also AX500), pin 13 of IC314 (LS139) needs to be connected to MA17 signal instead of GND.
This is done by a jumper J305 that can be seen on this photo:
https://www.msx.org/wiki/images/6/66/Yis805128r2f.jpg
For AX-350 there should be a similar way. The IC306 (corresponds to IC314 on AX-500) is there, but I can't see on the photos where pin 13 is connected to.