Hello,
Please excuse the answer in English, but my German is not the best.
I have some answers to:
[#6] The cover with a complete set of Mi 313-16 I is supposed to be a FDC - First Day Cover, since the postage stamps were issued 23 December 1991. There was no official first day postmark. Riga-52 was the parcel post office downstairs below post office Riga-50 right next to the Riga Central Station. It is obviously a philatelic make and the postmarks on the backside of the cover were not made by the postal service.
[#7] Postmarks with LV-1000 are from the sorting centre in Riga, which is situated right next to the Riga Airport. The acronym at the top - after Riga - is new to me. 99 indicates the postmark litera - the number of the postmark.