They are a fun rifle and you will enjoy it for sure.
1-Close 3/4x24 and yes they can be cut to 5/8x24 if that is what you want several Gunsmiths that provide barrels for us have this in their services and show what they charge.
2-Did you try to loosen the screws holding the skins together around the mag well. Just a half turn might take some pressure off the mag. (loctite if needed)
3-Are you asking about scope Zero and shift between barrels? My ballistics calculator allows me to enter the info and it adjusts my dope. Other than that drop the barrel in adjust my scope and shoot.
Enjoy post a pic when you have it all together!
inside the stock panels where the Mag release lever is you can physically spin it in and out ,
you can have more Spring tension so its stiffer to " click " your mag in or softer pressure .