If Kate has been feeling back to her old self for several days now I hope she goes on the trip, but I think the fact that they've pushed the decision deadline back indicates she's still feeling poorly, (or has been sick up until very recently). If that's the situation, IMO, she shouldn't go to Malta. The worst case scenario is Kate feeling 50%, deciding to travel, and then becoming severely ill again en route or during the trip. Then not only is her health impacted but Malta doesn't get any royal representative, everyone is disappointed, the press would lose their collective mind, etc.
However much Kate would like to go on this trip, I believe she and her advisors are probably very aware of the above worst case scenario and that the announcement, when it comes, will be that she's not going. I've been surprised before, though!