You are correct, Chas, about that window frame! It has never been completed. Several of my windows are the same, as I haven't put the trim on them. Yes, I have been slow about dusting that mirror! I did dust one spot just a little while ago, so that I coudl see something that I wanted to look at more closely.
That white stuff is a kind of a rubberized plastic. It is very flexible, and the add says it is completely water proof. So it should do the job. We got twice as much as we needed, then we took the extra back after finishing the job because we live so far away from town over rough, dusty, graveled roads, and we have to drive slowly, and watch for rocks to prevent damaging the tires.
Yes, our daughter did a good job with everything she did, and it was very nice of her to help us that way! I am sure she can mix concrete as well. I am not familiar with that term, "busdriver's holiday." We have a lot of expressions, but I haven't heard that one.
I have the curtain rod and shower curtain ordered, so once it comes, we will be able to be able to complete it, and then I can go to taking showers in it for real. Presently I am still taking showers in Carol's bathroom.