2001 Cadillac Deville Is this car trashed? Gaskets, Oil Pan

The northstars have had their share of leaks, the fact that they will fail again after repaired depends on the credibility of who is doing the repairs. As a Cadillac dealer tech, I have seen my share of repairs of this nature myself, and from many of my colleagues. Resealing the lower end of the engine has been a common repair for the oil leaks, and if head gaskets are leaking, it is repaired with a steel thread insert. The usual failure of the head gaskets is not the gasket, the aluminum threads in the block give up over time, to expansion and contraction of metals, and the pressures working against them. After a thread insert repair, I have yet to see a return failure in 12+ years, unless the repair was performd improperly in the first place. It s an expensive repair due to the fact that the complete powertrain(eng/trans) has to be removed from the vehicle, to be performed. If you chose to have the lower end repaired at the same time as the head gaskets, there is some overlapping labor that you would save on. As far as the oily, sticky, foam that is coming from the vent, somewhere in the HVAC case there is some sealing media degrading. This is either from something foreign getting on it, or perhaps refrigerant oil from a possible leak.
You make mention of a newer design bolt. There was a change made to the head bolts that occurred sometime around the 2003 era. It involved a more coarse thread pitch, that if had to guess, was made to combat the threads failing in the block.

I wouldn't say the Deville is on it's last leg, it is just in need of some TLC, and unfortunately it is some spendy TLC. If trading it off seems to make better economic sense, then it may be the best choice for your piece of mind.