TheBear,
You wrote “A gerus for marriage is invalid, period.” That is simply mistaken. While l’chatchila we don’t a geirus for marriage, everyone agrees that l’acher hamaaseh it is a good geirus (if there was proper kabolas mitzvos).
As Reb Doniel has explained, if someone does milah and t’vilah and is mekabel the ol of the mitzvos, it is a good geirus. This is true even if they know that it will be very hard for them to keep all the mitzvos and they will inevitably fail now and again.
Because dvarim sh’blev aynom dvarim we don’t speculate about what subjectively people had in their hearts. Rather we examine the kabolo of mitzvos objectively. The kabolo will only be invalid (and the geirus invalid) if there is a maaseh at the time that objectively shows in was insincere (e.g. someone is m’gayur at gunpoint, chas v’shalom). The machlokes in the Gemara about the kusim is whether their geirus was objectively insincere.