VISA rules when crossing CIS borders

This Easter I am planning to fly to Vilnius, then take the train from
Vilnius to Minsk and then train from Minsk to St Petersburg. I am a
Norwegian Citizen and I know I will not need a visa for Litheunia.

My questions a

-If I have got it right - As long as my passport and visa is ok, I will
not need a returnticket on entry?

-Can my Russian and Bellarussian visas have overlapping periods of
validity?

-Do I have to decide upon what bordercrossings to use when I issue my
visas? I know that I have to check if the bordercrossing in question
can be used by foreigners / non-CIS citizens.

Thank you in advance,

Jan