Originally Posted by Diesel911

LOL!
At idle the only pressure that could get by worn Valve Stems/Valve Guides/Valve Stem seals would be from the Exhaust side of the Valve System. At higher speeds even more so. (an exhaust restrection like a collasped Muffler could also raise the pressure.)

At higher RPMS if you have a Turbo the intake would also be under pressure and some pressure could leak by worn Exhaust Valves/Valve Guides/Valve Stem Seals.

Since the area under the Valve Cover is connected to the rest of the Crankcase by way of the space where the Timing Chain/Timing Gears are the pressure should be close to the same in the Crankcase and the in the Valve Cover. So no matter if Blow-by is causing the pressure or worn Valve Stem/Seals is causing the pressure or both of them are contributing to the pressure the space is connceted and the pressure should be similar.

Blow-by past worn Pistons and Rings as a cause of high Crankcase Pressure is just too well known to comment further on.

I do not know how it happens but some good Mechanics do not know how some of the things they work on function and some Mechanics know all of the Theory of how things work but are not good Mechanics.
This is why you need to learn the why and wherefores for yourself so that you can decide what is resonable when they tell you something.

Right of wrong my opinion:
Unfortunately the comment by the Mechanic on Blow-by not being the possible cause of the high Crankcase Pressure makes me suspect that the Mechanic was trying to sell you a Cylinder Head Job; something that he can do in his shop.
And, not a Engine rebuild job that would fix the Blow-by problem; that you would not go for because it is too expensive or it is more than he could handle.

If you want to you can find out if the Valve Stems/Guides are worn without removing the Cylinder Head. The FSM has instructions on how to do so.
To do this you should pull of the Valve Stem Seals from the ones you plan to check (you do not have to check all if you find a few are worn beyond spec.) but you would need to buy a set of Valve Stem Seals as after removing the old brittle ones they probably will not be reusable.