After the recenish renovation of several bathrooms I would say that all the "extras" you have stuck in the corners are totally supurfluous.

When shower recesses and other wet ares are waterproofed asingl layer of ,I guess, liquid rubber is put down. No tape or anything else in the corners.

Looks like you have a job in front of you to retrieve the situation.

It may be possible to inject liquid rubber under the folds using a syringe.