moishepipik,
You are confusing halachic categories. The Rambam holds that B”D is koifeh by a legitimate (i.e. with amasla) maos alai right away. Rashi holds that by maos alai we give them 12 months to work it out, living together, and then are koifeh if they can’t work it out. The Rema and the Mechaber pasken not like the Rambam and Rashi, but rather like Rabbeinu Tam and the Rashba who hold that we are not koifeh by maos alai because “shema nasna ainav b’acher” and if the maos alai was a lie, then the kefiah would be improper and the get posul (note that it is a chashash mamzeirus, not actual mamzeirus). However Rabbeinu Tam holds, and the Rema paskens, that societal pressure is not considered kefiah at all.
There is a separate din that if a husband is no longer living with his wife and no longer supporting her on a regular basis, then we are koifeh. What Sam2 and I were describing is this latter din.