Seems like you've had rotten luck. I've seen others take their units apart with no problems and my own unit has had no issues so far. I haven't taken it apart yet, but from what I feel of the unit so far it's built well.
If I cannot even play Super Mario World as effectively as my JXD S7800B or GPD XD / GPD Win 1... How the heck can I justify spending more money and time buying a THIRD GPD Win 2 unit?! You guys need to beware! Many sellers on EBAY selling their GPD Win 2 units are doing so ONLY because of this BUTTON issue... Multiple buttons being pressed down will lock and cause a grinding/cracking/rubbing sound against the unit's shell and holes where the buttons protrude. Seems the ABXY buttons are nothing like the shape of the old Win 1 unit and they are taller and uneven causing this issue... Such crap... The A/B Buttons are the worst with this issue. The X/Y buttons are not that affected.
I bought 2 units and had thought about buying a THIRD in hopes of getting a proper one... I even bought one that seemed to be taken apart and modified... Now I see why the previous owner tried to modify it... He was trying to fix the DPAD and BUTTONS... This is not worth time and hassle if you play older games that need multiple buttons pressed at once... Games like Mario/Contra will suck and not be playable as they should be! Just a heads up guys.
Yeah, I wouldn't buy Chinese devices like this off eBay unless it's mega rare or something (like an iPega PG-9501, anyone have one of those?). Anyway, I've no issue with my unit's ABXY buttons at all. They're not uneven, and they're not poorly cast either. All the tolerances on my buttons are perfect and I have no issue playing classic Mario games with these buttons. You seem to have been unfortunate enough to receive only units from some bad batch that they made at some point.
That said, I do have issue with the DPad for Mario games, but that's the same issue with all of GPD's current products. I'll likely replace the membrane and the actual d-pad once people figure out the perfect replacement combo, like how a good replacement for the Win 1's dpad is the Wiimote membrane with the 3DS XL dpad.
Either way, I barely play any dpad intensive games anyway, so I'm not exactly in a rush.