I won’t belabor that point, because true polymorph exists, and if this character has that ability it can be assumed that’s the spell he means to use. If he doesn’t then it wouldn’t work if he tried.

All rules CAN be changed, but they need a good reason. This guy is trying to cheat the system (skipping a will check). “I said some words but didn’t actually get consent to cast the spell I’m going to cast” isn’t a good enough reason.