Of course they could. And Rome would back them up on it.
I doubt it. But I tend to think that most pro-capital punishment Catholic politicians would not put up much of a fight on this one. They would more likely have an "Sigh, okay Your Holiness if you really insist but I am not excited about it" attitude. People who are pro-capital punishment believe in following the rules and they respect authority. They would, if pinned down by the pope, probably give in. And anyway, putting someone in jail for life is satisfactory in most cases. Not only that but there is so much wiggle room on what "rarely" means or "grave" crimes that the whole thing is harder to pin down. I am not worried about it.
"As to the Pope’s assertion that the death penalty should today be rare, .... this is to be understood as an exercise of the Pope’s prudential judgment. “Prudential” has a technical theological meaning .....It refers to the application of Catholic doctrine to changing concrete circumstances. Since the Christian revelation tells us nothing about the particulars of contemporary society, the Pope and the bishops have to rely on their personal judgment as qualified spiritual leaders in making practical applications. Their prudential judgment, while it is to be respected, is not a matter of binding Catholic doctrine. To differ from such a judgment, therefore, is not to dissent from Church teaching."
Avery Cardinal Dulles, S.J. Fordham University Bronx, New York