think of the mechanics involved - if you have valves that are stuck shut, you've got major problems!
A valve can't stick closed without a push rod being bent, a rocker arm being broken, a valve stem bent real bad - etc.
It CAN happen, but unlikely enough to go much further down on your "things to check" list.
Sticking open is a different story - but then compression wouldn't be there.
MUCH more likely that your carb is the problem.
I may sound like a broken record because I always reply with this answer but...
Spray some starter fluid in and see what happens.
If it fires, even just a little, you can be pretty sure the problem is in your carb.
(yes a weak spark may still fire on the fluid and not gas - but that's a relatively rare situation).