Pushing the diverter down after the water is turned off will still allow the water to drain from the shower arm to the fill spout. There is probably some roughness in the brass castings that is causing increased resistance, but that shouldn't be a problem when you turn the water off first. It may work easier after repeated use.

Is anyone able to tell me what brand is this shower faucet

Most showers are limited by code (and just common sense) to 2-2.5 gpm. In Cal I believe it's 1.8 now. Yes, your GC should know local code... it's part of why he's getting paid. Forgiveness is definitely not easier than permission if you get flagged. This may be your first rodeo, but code inspectors have seen this a million times already and are looking for it. You start getting into water heater size and other issues when you go with multiple shower heads also. It gets expensive, and the novelty wears off pretty quick also. Be careful also about getting too far afield of what your contractors are familiar with. If your GC is wise, he's giving you some supplier choices that he's familiar with and can support. You don't want to learn about "issues" with odd-ball faucets at finish. You want to stay in your role as "Owner" and not muddy the water by getting too much into GC territory. It's the GC who is responsible for it all coming together right. It's fine to say what you want the end product to be, but when you get into flow rates, psi, code, etc... you are in the wrong lane. Most of the major US brands over a certain price point are going to give you long life, warranty, etc. It becomes about style and such after that. I think most of the valve bodies for Delta, for example, are standard.

Thank you so much for responding! Hopefully plumber can find something which fits. I have a sinking feeling someone renovated and added tile to existing fittings which made the screw-on spigot ( if that’s what it’s called 🤷🏼‍♀️) to short for the tub spout to catch fully. Fingers crossed!