Yes I suppose at that time probably they thought things were never going to go this way . So thai immigration is now cool about every one hanging in till 31 July
Firstly what is the letter for ? Second there’s an automatic extension till 31 July . However just to be safe keep a record of phone calls you made just in case
You can do that . Having a boarding pass means you are checked in and if you don’t have luggage why would you need to see the airline counter anyway ?. I ve done it many times with thai airways coming into Bangkok .