There seem to be two problems here.
- A marriage visitor visa is only appropriate if your partner intends to marry in the UK but leave again afterwards. Immigration will be very suspicious that this is not your intention. The appropriate Visa for marrying and then living in the UK would be a "Family Visa"
- An immigration officer is going to be very suspicious about someone wanting to marry someone they have never met in person. The sad fact is that "marriages of convenience" were/are an all too common strategy for evading immigration rules.
In my opinion, if you are serious about this relationship you need to start seeing each other, both to satisfy the authorities that your relationship is genuine and frankly because a Marriage or Civil Partnership is a long term commitment that you shouldn't rush into lightly.
I am not an expert but I doubt a single visit will be sufficient to establish a genuine relationship. I expect that to avoid serial refusals it would be a good idea to talk to a lawyer before making any further UK visa applications.
If your proposed partner can't get a Standard visitor visa to the UK then it seems like your only real options are to either meet with him in Russia or meet in a third country.