While the economics can change based on trade offers, I think Mannion is safe - he is the only QB on the roster who is under contract for more than 2016, so he will at least be a cheap backup. Keenum is the best starting option on the roster - and Fisher will want a solid backup for the rookie. However, his money is not guaranteed, and the 1.75 million Foles is yet to receive is. But I suspect they will get whatever they can for Foles, although it may not be until training camp or at least close to it, and it may only be a conditional 2017. And if nothing is offered, they could keep Foles around in case of injury until later in training camp.
If Denver offers enough for Keenum, especially before or during the draft, the thinking could change. I think the Rams would take pick #63 for him now, or even several mid round picks to allow holes to be filled.