Congrats on your settlement in the near future. Larkin! Hope you find the place of your dreams. It will mean lots of work to get it where you want it, but it will be worth it.
I'm super glad that Houdini is enjoying his new digs and that you found food he enjoys. I know the other baby snapper I found the day before Houdini came (if it was a different one, that is) also had the same type of shell, so I think that is normal for this area. I know the red slider has a softer shell and it was semi-hard but he was bigger.
And, with his "disappearing" skills, now you know what I was going through trying to locate him for hours here in my house, having no clue of where he may have gone. At least now I know to never give up, and never assume the cats ate him, I'll look forever next time I have a critter get loose in my house. It's very possible he was always in the sump pump pit, and I just simply did not see him. I know I remember reaching in there and feeling all around with my fingers the first time, though. It's still possible he was snuggled up against the pipe on the back side and I didn't see him. Anyhow, glad he is enjoying the plants, etc. in his new environment.
And, you didn't say, Larkin, but if he needs sunlight to harden his shell, will an aquarium light do the same thing? I know when I had a little slider, the pet store told me it needed a bright light that would put out a small amount of heat, so it could get completely dry every now and then, but that is the red slider, and they are always on logs, etc. I still have that very costly light, too. Stupid me bought it at the pet store. LOL Now I know I should have gone to the farm and garden store and gotten a heat lamp shade, and then used a regular bright light bulb. Live and learn ....