Heading towards CIS-2020.

I'm just worried about the situation with refunds if none of the above restrictions apply: the airline and the hotel in St. Petersburg won't refund my money.

On what grounds won't the hotel in St. Petersburg refund the money? They are obliged to refund it if you request it right now.
As for the airline tickets, which airline is it?

Kirill, resorts, sanatoriums, and recreation centers are closed until June 1st, if anything.
Throughout Russia (in the regions, they are still trying to stretch things out, as the wording of the government decree allows for various interpretations, but the final result will be the same - everything is closed).
Last night, they started evicting everyone from hotels in Sochi.

Postponing the CIS summit to August (or later) is no longer just a guess or a backup plan; it's the only possible option in these circumstances. Therefore, reschedule your booking and ticket dates to August if you plan to go.
Refunds are more complicated, but also solvable: get a sick leave certificate and receive a full refund.

P.S. I am amazed by the naivety and stupidity of our citizens. I personally won't go out even to St. Petersburg, let alone anywhere else, but a bunch of idiots have rushed off on trips to visit relatives and friends for barbecues and other things (spreading the infection throughout all regions).