So much wrong with what you said and what the builder said. You should have selected a shower waterproofing approach a long time ago. You need pre-slope and waterproofing. Food tests. Some kind of collar for the tub's drain line.

That tub has anti tip brackets. How do you plan on securing them into the waterproofing.

Are you using a capillary break. A baby dam? Can the shower side hold an 1" pf water above the drain top before you go Oh _____!!!!

I do mini design calls to help people better understand this process and the key points that need resolving.

Check out more here: Understanding what isa shower design call

They haven't laid down the waterproofing membrane or the concrete for sloping. There will be a ledge between the hardwood floor and shower area. I'll check with the anti-tip brackets, as I thought since it is mounted into the wall, they wouldn't be necessary, but I guess the other three sides are free. Thanks much for the info.! Will check out the link.

How did this tub work out for you? Is it something you would recommend?