[alikhan0]

Salaam, Salaat in the masjid is a fard kifaya, meaning, a communal obligation, meaning, their must be a group prayer in the masjid/mosque of a particular area or else the whole community is sinful. And if by chance one prays there (may Allaah allow us to pray every prayer in jama^ah) then the reward is multiplied. If one prays at home, alone, and the communal obligation was fulfilled by other than him, than this is NOT sinful and affects the rewardability of the prayer, not the validity of it.

Also, there is difference of opinion about the salaat and kufr point you raised, nonetheless, even those who said it is blasphemy to not pray, said if one does NOT pray, not if one prays alone. Again, one is only sinful for praying alone at home if the fard kifaya (communal obligation of prayer in jama^ah) was not fulfilled.

One can pray jama^ah with a brother, sister, mother, father, friend, cousin, wife etc etc at home and receive the reward for it.