Well the "maximum weight" thing does only apply for Star Alliance and for flights included in the itinerary.
For the separate ticket you still get the *G benefit if it's on *A, the second non-*A airline probably won't care what other tickets you have and will apply the 15 kg rule.

Do you really need to carry your luggage with you all the time? Can't you leave a suitcase at the destination where you're taking the domestic flight?

It's very hard to give a good answer when we don't even know what routes and airlines we're talking about.