Some of the solder blobs on the backside are a weeee bit big. It's hard to see on the photo, but there are several spots that look like they could be shorts. Apply some flux and remove excess solder with some solder wick. Mostly in the P,Q and L,M columns. And then clean it up as suggested with ye olde solvent + toothbrush. If you don't have rubbing alcohol you can use something more aggressive in a pinch, as long as you clean that up as well. Some warm water with soap works fine for that, and then blow dry it with the human powered blower. Of course using IPA is nicer than say kerosene, but just suggesting that you don't have to be too picky about what solvent to use for a first job since you might not have collected all the goodies yet. After you get this job done you can get some IPA for the next projects.
About that positive rail made of solder ... to avoid that you can use a piece of fairly thick solid copper wire. Strip it, pre-tin it and then use that as a power rail instead of the mountains of solder. Or if you have some cheapo bare copper clad, cut a strip of said copper claid, super clue it to the perfboard and use that as power rail. Also makes for easy mounting of some decoupling caps where needed.
And it indeed looks like the manufacturer messed up the numbers on the top vs bottom side. "What? Spend 5 more minutes? Copy/paste silkscreen."