Yamaha CX7M/128 (MSX2) key is stuck

Can not find any picture of the disassembled keyboard for this model. In general please do not put anything inside the keyboard. No oil, no glue, no lubricants etc. And yes, to fix it properly (now when you put lubricant inside) is to disassemble the keyboard and clean it. But you can try the following. Looking from the standpoint of your picture you can see "ears" to the right and to the left holding the key at the top. You can see thin holes to the top and bottom of these ears from both sides. In these holes you can see small latches holding the key from the bottom. If you take two of something thin and flat (e.g. screwdrivers) and carefully insert into top and bottom holes at one side, pressing these something (screwdrivers) NOT in the direction down but in the direction towards the key body, you may be able to unlatch the key from one side. When unlatched one side, do NOT try to remove it, keep it slightly unlatched perform the same operation from another side of the key. Only when key is unlatched from both sides you may lift it up, and then disassemble it to see what is wrong. Key does NOT require any lubricant, and if it does not work, it means key is damaged or something inside it prevents its proper operation.

Disclaimer: the part is plastic and you will do what I said above at your own risk. As you said, the best is to disassemble and unlatch key from the inner side of the metal sheet holding the key.