I would try to get something in there like the dental tool and loosen it. Maybe brake it up into small pieces shop vac'ing with a crevice nozzle at the same time. If that doesn't do it, at least if you could drill some holes in the foam with a long bit to make channels, that could work as well. I think sill pans are most critical with brick veneer and stucco exteriors, probably not so much with lap sidings. If your windows are flashed properly, you might be okay unless you have brick or stucco, then you definetely need a place for moisture and water to channel out over the sill pan.

I had to do some temporary fixes with windows that were rotted that looks similar to yours. I just had to wait until I could afford it. I used that wood hardener like you did. To fill in the void after digging out all the rot I used whatever, small pieces of wood. I would put some wood, or something solid under window to keep it supported. I also used some spray foam, use the stuff for windows and doors, it is not supposed to expand so much. I also used some wood putty (not the stuff used for filling nail holes) There is a wood putty that is ready to use, I think made by the same place that makes wood hardener. The other wood putty, it's a powder that you mix with water, Durhams. I then painted/primed to give it more protection...I had 5 windows that looked similar to yours. When we got enough money together to replace we had to replace the bottom framing and a stud or two on a few of them. Yes, I do think the sill/bottom is a support. They are nailed into the side, but the bottom I am sure is "what holds it up." Also, I spray some bleach around the area to kill mold spores, I read somewhere a mixture using borax is good for that too. But after wetting it with the bleach you need to dry it out really good before you seal it all up. The two windows that I had to have studs/bottom 2x4 (that supports the window) replaced I had to tear out drywall and dried it out using fans and portable heaters.

What I'm saying is that a sill pan would not be visible in a completed installation. I agree with Joseph's suggestion to water test or possibly view other units. There is really nothing to say that your installation is improper, nor can it be verified that it is correct without pulling things apart, at least not beyond what can be seen. Water testing would offer some assurance. Given how new it is, you could check in with the building department regarding their inspection also. What I find in most cases is that home inspectors, many of whom are unqualified to do their jobs, will flag things unnecessarily to try and justify their fee.

Remove it and put in a wood sill. Those tile sills are not a nice look.