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 would expect all my window stops to be the same dimension. Most of my windows are wood double hungs, newer windows built to mimic the old ones that were here but with true divided light. So they're a more modern installation, but with the old charm. I just walked around and looked at them - I have a few casement ones too - and they all have matching window stop dimensions on all four sides.

The existing sill appears to have a seam where it meets the window. If so this would make removal and replacement fairly easy. I would cut the new sill at about 8 degrees and raise it the bottom of the window and if there is enough room run a piece if flashing under the window and past the joint. I would also back caulk it and shim it . You could also shim it with a piece of cedar beveled siding and even cap it in coil stock.

@Beverly good question as to whether they are decorative (how can you tell?). That said they are still falling apart and it does not look good. It is actually a mason who did it last year… to correct the condition in which they were (they were starting to disintegrate but it had been about 10 years).