Yes the RCS chats work over wifi, but that only works with other devices that have it turned on. With the Pixel, all picture and group messages were able to download over wifi and it didn't have to come from a phone with RCS turned on.
I was able to send MMS messages on my Pixel 6 with the data turned off and the wifi turned on. I have never been able to do this on any Galaxy device and that is one of the only features I loved about the Pixel.
I used the Google messages app on both devices.
Yes, but that shouldn't stop you from being able to merge all of the old text messages into the thread of the new number. That is just a huge design flaw by (Samsung in my case).