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.

+1 on handymac - lumberyards would carry 1 3/4" x 7" for a 2x4 wall and 1 3/4" x 9' for 2x6 walls and they are most generally made of Ponderosa Pine.

@Darlene. You may wish to start a new thread. There are several professionals here willing to help. Yes you can fill a small gap with setting mortar depending on the mortars limitations. Some form of backing or reinforcement will be needed. As I stated above always go directly to manufacturers for installation instructions. Yes showers have a high degree of difficulty with a lot of details that must be followed. If you choose to proceed please purchase a TCNA Handbook. A $35 investment well worth it. You must use caution when seeking advise nowadays. I have found YouTube will have more bad info then good. Again go to the manufacturers videos! Unfortunately Handy Huber with his 1 project experience has resorted to name calling while providing you with bad advice. "then you have to run the cement board vertically from bottom to top." FACT Cement board is installed horizontally with staggered seams. ANSI 108.11 page 57 A-4.8.5.1.4 Cement board recommendations such as "new piece so that it's snug" "nice and tight." FACT The manufacturer instructions state leave a gap. https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-durock-cement-board-with-edgeguard-installation-guide-en-CB237EG.pdf "And most importantly make sure when you get to tiling that you use trim tile ALL around the window frame! It's a crucial extra layer of protection against the constantly wet conditions and protecting the seams." FACT Tile has nothing to do with waterproofing. If you can't run the shower without tile it is not constructed properly. Many feel as I that showers are not a DIY project. Can it be done? Certainly. It's all in the details. Good luck with your project.

Agree with @hallettco can't tell anything from these pictures. One shows exterior siding with what looks like composite board butts to it? The other one it looks like composite board overlapping sheetrock if that's the case they should have used drywall end-caps, to get a nice, straight transition they come in plain or beaded corner something like this Or you can get a low profile edge cap at this point and paint it something like this or Drywall end caps is the way to go, the molding options just to cover up what you have now. Talk to your contractor ask them to slip the drywall end caps in they should be able to do that will give a better finish. Good luck