On our trip there were at least 4 people traveling "solo" but Viking has been upsizing their ships and in the process eliminating single cabins. I had a single on my Ukraine trip and it was great. River cruises are pricey as is, without paying almost double as a single.
If your trip is through the Three Gorges, you will encounter some overcast and some drizzle just about year-round. It's the terrain that causes it, not the season.