IMO, you need to stop and back up to do some research. Your tile setter does not appear to be up-to-date on current products and methodologies in order to create this. Personally, I would never leave it to a tile setter or plumber to create the plan to do this - I would want a pro involved. The last thing you want to do is work with someone that is going to learn on your project.
It is more difficult in a second floor to create a barrier free shower. There are now systems on the market to create this without cutting down the floor joists - which historically has been the way it is done. They are fairly new in the scheme of things, so none have been out long enough to say they are holding up over the long term. Go to some reputable, professional tile stores and ask them to explain the process and possibly point you to some videos online to review. Once you have a better handle on your options, have them give you the names of people who have done this, are experienced and warranty their work.
It is not sold in our area (it may be in yours), but the system by Trending Accessibility is one we have been watching because we think it shows good promise for holding up well over time. We have not tried it, but it looks very nice. They have some good videos on their site that show the process - look for the flexible fusion shower pan video. We have some appointments with some local tile shops to look at the products they market that are supposed to be comparable to this - but I don't know the names of those products.
Good luck.
Q