I have read some threads here telling how to "cheat" the system and get benefits as much as possible.
In practice, it looks like in this way:

1: a *G & MM silver passenger uses it's Gold card for check-in at the C or F counter and gets the priority tag for its bag. The agent will register the *G number for crediting miles.

2: access to the senator lounge using the *G card.

3a: at the gate, ask the gate agent to change the miles crediting from *G to MM silver.
OR
3b: Log in to its MM account or booking management and change the FFP number to the MM silver for miles earning.

Can this really work? Has anyone done this?
Is step 3 possible or feasible ? 3a or 3b is better?
Any better suggestion?

Thanks.