I picked up my car from the mechanic today, after replacing the valve cover gaskets and sealing the breather covers with silicone. So far, the oil is not leaking, so it looks like the job was done correctly.
For those interested, in Poland this repair cost me $125. And how much would it cost in the US, for example?
At the same time, they replaced the rubber hose for the engine breather.
AutoSerwis gives me a 2-year warranty for this repair, which is the same as the warranty for the gaskets.
And as for re-tightening the bolts on the valve covers, the mechanic said that in his opinion it can be done, but it would be better after a year.