It was much closer than you would think. It just came about that using the 2 seat airframes was cheaper as they have been in production for decades.
The F-15EX might be capable of multirole, but that is not their use in the USAF, at least for the time being. They are to replace the F-15C/D fleet that has reached its airframe end of life. Just because the jet can do it, does not mean it’s worth the time or money to have pilots trained in both roles, when their purpose is to be the best at one. Just because there is a second seat doesn’t mean its occupied.
No. The beauty of the F-15 in relation to the 5th/6th generation aircraft is the ease of maintenance and aircraft availability. Slathering the entire thing in RAM will have the opposite effect, while having almost no reasonable decrease in observability as the F-15s structure is just not good for stealth. The intakes are just flat out horrible, as is the nose barrel. Canting the verts helps a bit, but it doesn’t change the fact that most of the aircraft is a very good reflector. Couple that with the requirement for internal storage of weapons kills one of the major benefits of the F-15 which is magazine depth and combat radius, because you incur the bulk of the CFTs, with none of the gas, all while having less missiles.
It also just flat runs counter to what the F-15 does best which is DCA. It’s going up and saying," hey, I’m big and I’m bad and I’m rolling around with the Eye of Sauron. I see you and you know I see you so you’re not going to do anything I don’t want you to do, or else." If the F-15Cs are going up, they aren’t trying to hide. They want to be seen, and they are making their presence known.