Is the head gasket supposed to block coolant passages?

I look at it as possibly that a radiator stop leak was used at one time or another which caused sludge to build up.
I too have had issues with head gaskets that seem to obstruct some of the passages. A good flush and back flush of the entire cooling system after you get it repaired is warranted. I have seen this on not only VW engines that I have worked on Diesel and Gassers but as well in the parts yards where folks removed the heads off of engines.

Using the proper coolant and Flushing and backflushing the cooling system every 2-3 years sames as bleeding the brake system insures that you are at the maximum coolant flow, and that the rust that may be occuring in the block is washed away... I used a Prestone product on my Diesel a few times over 20 years of driving it usually before a WATER Pump change or t-stat change, as it not only clears sludge out of the engine but the back flush also cleans out the radiator and heater cores.

If you have a partially blocked heater, a good flush with CLR on the heater matrix only and a back flush will usually increase the heat....