Rats are fantastic at hiding pain. It's in their very bones to do so. Lymphomas cause pain one they reach past the size of a nickle. It's a growth that shouldn't be there and it pressures their body, it's heavy too. There is no way around it and even if they act fine just remember how damn well rats are at adapting. I have to honestly state the rat breeder is way off, a baseball sized Lymphoma is nothing to be 'fine' about and it seems you have some very snarky vets. Either way Lymphomas are painful and they do drain the rats of energy.

If you feel your females can't handle surgery (though it sounds you don't want to do it regardless) then I think your responsibility to put them down. It sounds to me like the Lymphomas are taking their toll.