Regarding your questions:
1- if I choose to ignore the CA, should it increase/decrease depending on the events in the game?
Yes, since the freeze feature will ignore it and wont touch it.
2- if I choose to freeze the CA, but check the "attributes can be increased" checkbox, should the CA increase/decrease depending on the events in the game?
In this case CA can only increase depending on the events in the game FMRTE wont let it get below the value you froze it, so if you froze one player with CA off 180 and as the time pass the game tries to set it to 175 FMRTE will correct it to 180 again, on the other hand if the game tries to set it to 185 FMRTE wont touch it and will let it fluctuate above 180.
Let us know if you have any other doubt