That alcove is designed to house a tub only, not a tub shower unit. You can't "repurpose" what's there without water damage to your floor, window, and behind your walls. The tub itself not designed to be used that way, and neither is the window designed to be in a wet area. You'd need to rip it all out and start over by installing a smaller and waterproof window, then installing a tub that had an integral tile flange which is part of the waterproofing system needed. Then your waterproofing membrane and whatever wall cladding you might want.

Agree with above posters - look more toward what is the norm in your specific location . Where I live, a very urban residential area, not only do some of us not have garages but we dont have a driveway or offstreet parking of any kind. Yeah, that means our properties are valued less, but that can be a good thing - means I can afford to live in a neighborhood I couldnt otherwise afford - and then when its time to sell, Ill pass the savings on to the next person - and I expect there should be a lot of interest because of its affordability. Anyway - someone local could give you a better sense of it. Around here, offstreet parking is highly desireable but a garage is not a must. In fact I can think of one nearby residence where the humongous garage has been a liability (placed in their front yard bc there was no other location for it).

(not a pro) When we did a big reno, we added a 2nd hall bath on the 2nd floor (so we have 2 halls baths and the master bath on that floor, serving 4 bedrooms.) I have one girl and one boy and when they were in middle and high school, it was very helpful that they each had their own bathroom, for expediency getting ready at the same time and for storage/privacy (they have different "stuff".) (They also learned how to clean their own bathrooms.) Also, I'm going on the market soon and having the second hall bath definitely helps differentiate my house from houses with only 1 hall bath upstairs. (I also have a full bath with guest room in the lower level for the occasional guest.)

I do not see a correlation on the size of your family or the kid's ages influencing a walk-in closet in the Master Bedroom. If your house is a true original Victorian, then you are certainly aware of, and resigned to living with, a lack of closets from that era. Making a Master ensuite is a necessary purpose, however, I cannot imagine that capturing the turret space in any Victorian would be an appropriate solution for a good ensuite layout, closet functionality, or ROI. Reserved on seeing entire plans of the floor(s), but I would lean towards your alternative solution.