Am really sorry to hear of the bad news, I'd been following this thread(my jumbo Straw just molted too and he is pale) and only just found out that he(your Straw) didn't make it.
Strawzilla is usually iso'd for around 2 weeks after he molts. More if he needs it. You can tell when hermies are ready to be placed back in the main tank because they are running/walking around on the surface, otherwise it's best to leave them be.
Just looking at the last photo, it looks like your Straw had 3 leg tips that were very dark(almost black). Were the legs actually dark or was it a trick of the light in photo?
If the Straws legs were actually like that and the rest of him was very pale pink/orange, then I'd suspect some type of bacterial infection.
I wonder if it could be something due to the substrate?
actually had this happen to one of my Aussie crabs more than a year ago, where one of its legs turned black-going about half-way up the leg.
After a week or so, the crab dropped the leg and not long after he died.
Odd thing was that with the dropped leg, I crushed the black exo and instead of the inside being gooey and white, it was black and crumbly.
This can happen due to injury or a molting mishap or possibly a bacterial infection.