Xbox one gamepass games download query

You do not delete any games to use them digitally with Game Pass. I have two Xbox's that I use and I have never needed to delete a physical game from my HDD in order to play it on gamepass. I am unsure of your second question because I know download
sizes are different for MW depending on if you have a disk or a digital copy.

When GTA V became available on GamePass i traded in my physical copy and clicked on the GamePass version, it did a quick update and i was ready to play. My assumption is that the Game will install from the disc and the disc has to be in the console to
verify that you own the game. As soon as it was available on GamePass so long as you have GamePass you can just click to play it all the same. The way to check this is to simply take your game out of the disc drive and click on it to play and see if it works.
If it does an update just look at the size of the update. If it is just a few GB then it's merely an update of some sort but if its a full 70GB then you know it wants to reinstall the game (but that didn't happen for me).

If you don't have the physical disc it prompts you to purchase the game.

If you cancel game pass you will loose the ability to play all Gamepass games unless you have purchased them separately from Gamepass.

If you have purchased Forza Horizons 4 then you should be able to see your purchase in your history and query with Xbox support.

Gamepass has always been an xbox one feature only

Any games with gold or purchased seperately is a seperatr download

you can only get games from gamepass within the xbox one as it still requires the gamepass app

If you make the purchase the game or games with gold then you still get to keep them