Let's not confuse a soluble carboxylated silicon compound with silica gel. They are two different things. Amongst some of the uninformed nonsense I have read on this subject is an opinion that silica should not be used because it is abrasive, and indeed silica is abrasive, but silica is not what is added to antifreeze. Silica is not the same as silica gel which is not the same as a carboxylated silicon compound.
The soluble carboxylated silicon compound is a corrosion inhibitor that reduces the electrochemical reaction between iron and aluminium. It is not solid and therefore can not block coolant passages. (Incidentally aluminium has been used in engines for many decades before 1996.)
Silica gel beads on the other hand are used throughout the commercial and industrial worlds for absorbing water, which I presume is their function in the expansion tank, because most hydraulic fluids are hygoscopic, i.e. they absorb water which degrades their performance. For example you will often find small bags of silica gel beads in the packaging of steel and iron goods to protect them from rust during storage by keeping the atmosphere dry in the pack.
None of which gets us any closer to solving your lack of cabin heating. I don't know if the absence of a Mit Silicat mark can be taken as proof that there is no silica gel present, so you have nothing to lose by back flushing the entire cooling system in stages.
As the scale of this silica gel problem becomes more apparent to me, I now wonder if some of the reports we get on the forum of unexplained low engine temperatures are related. I imagine that these beads can easily prevent a thermostat from fully opening or fully closing. They could also be clogging the coolant pump, which of course is designed to pump liquids not solids.