I've found how to change the pokemon's Nature, and thought I'd share it in case it's helpful. It's a little fiddly, though. Maybe this info helps to integrate it into the table
In Cheat Engine, connect to mGBA as described above, and Get Start Address. The address of the first pokemon slot is also its
[Link]. Right-click this address in the table and choose "Browse this memory region". In the bottom-half (with all the hex values) you can now overwrite the byte at this address (it's in the top-left corner).
In the game, check what Nature your first pokemon has, and note the corresponding number in
[Link]. Let's say it's Modest (15), and you want to change it to Adamant (3). This means that you have to subtract 0xC (12) from the byte at the address of the first pokemon. Because 15 - 12 = 3. Make sure you subtract the value as Hex, not Decimal.
This can cause the Gender to change. To change from Female to Male, keep adding 0x19 (25) to the byte until the pokemon becomes Male. To change from Male to Female, keep subtracting 0x19.
To change the Nature of a different pokemon in your party, just move it to the front, and repeat the steps above.