1. I applied the solder paste to the other side and aligned LEDs (polarity!)

2. Then I aligned switches. They are having small pads, so I had to check if they got wet with the solder paste. 

3. Then I heated my PCB with the hot gun the same way as in the previous log. It turns out, that I applied too much solder paste. All LEDs got bridges. So I repeated my previous exercise with flux, soldering iron, and even used a desordering pump. In the end all LED pads were separated.

4. It turns out, that I forgot one via, and one of the red LEDs left not connected. So I added a wire made from an old ethernet cable. Yes, it looks like a log, but it was the only not stranded wire I had.

BTW I fixed this via in the published gerbers.

I attached the battery and voilà. It worked.

Interesting observation. When I fried this side with a heat gun, the tin on the other side possibly melts, but the surface tension of the melted tin was enough to keep everything in place.