During reinforcment phase can the Soviet player build new blocks in key cities or only in soviet eastern links?
You can build them on Key Cities.
The printed reinforcment squares on soviet eastern links have the same use as the strategic reinforcment key cities?
I don't really understand you question. Those "cubes" that are provided by the eastern links can be used to "repair" blocks on that front. If you use them to buil new blocks they must be placed or the eastern link.