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I think you're missing the most likely possibility:

3. Russia is operating pro-Ukrainian bots to influence pro-Ukrainian people.

The bots pretend to be pro-Ukrainian but then try to influence the public opinion in the ways that are useful to Russia.

Why is a scenario that needs the requires the most assumptions the most likely?

The scenarios 1 and 2 also require a lot of assumptions, you just take them for granted.

I don't agree that my scenario requires more assumptions that scenarios 1 and 2.