Seems apparent that the release push button is not moving the release quite far enough to completely disengage it from the forend lump. I don't know if H&H forends have any adjustment for this, or if you will need to have someone examine the mechanism to see what needs done to fix it.
Prying it off with a screwdriver is apparently providing additional leverage to cam the release back a bit more to finally free it. It is probably a very small amount of additional movement that is needed, so everything looks OK when you operate it off of the gun.
Using a screwdriver to pry it off should be just fine until something breaks, or until the forend lump solder joint fails.
