After 100+ tests, here's what actually works
Use Mercuryo directly, bypassing SimpleSwap entirely. This uses SimpleSwap's widget ID temporarily.
Run a server that uses Playwright to automatically click Mercuryo for users. Technically possible but complex.
Guide users to manually select Mercuryo on SimpleSwap. Not automated but preserves SimpleSwap branding.
After extensive testing with Playwright, analyzing SimpleSwap's source code, and trying every possible parameter:
The only ways to achieve automatic Mercuryo selection: