If Newt and Paul are ineligible on the first ballot, “their” delegates all become free agents (technically, “unbound” delegates, akin to “super-delegates) on the first ballot. If Romney is close to, but not over the mark, they would be available, along with other “unbound” delegates, to get to the mark. Newt or Paul could act as kingmaker, working with enough of “their” delegates to put Romney over the mark.
Once over the mark, Romney is declared the “presumptive nominee” and everything starts being run by the party based on this presumption. This means speaking slots at the convention, the party platform, and as of yet uncommitted superdelegates. Even already committed superdelegates will be under pressure to switch to the presumptive nominee.
I don’t think Newt will be a problem; but, you can never really know about these things. Paul, on the other hand, might be looking for a legacy.
I believe Newt would support Santorum before he ever would Romney.