As you have noted there is a limit of 100K transferred per account. The Marriott Rewards T&Cs do state "A Member can receive up to a maximum of 500,000 Points per calendar year from other Membership Accounts". So you should be able to transfer up to 100K each (total of 300K) from the other 3 family member accounts. So you should be able to do so without any major issues and still have room to receive more points from different accounts.

Not sure AM (i.e. after merger) but historically, point transfers haven't updated the expiration clock. But do note that redeeming points for awards does update the clock. So if you move all those points into a single account, just make sure that at least once every 24 months the owner of that account either does something to earn points or uses some points for an award. Also, if that account has a Lifetime elite level, then I do believe the points expiration may not apply, but not sure if that is still a rule or not. So don't hold me to that one.