I just signed into this forum and got a lot of help with some issues that I have been having, so I felt like giving back. Unfortunately, I don't know if anything I can say will help you out, because what you said didn't work for you, did work for me.
I too was having trouble viewing videos in DS Photo +. I tried just about everything I could think of. I downloaded several video converters and converted my videos to a file type that I knew would play on my iPhone 4, but none of them worked on the DS Photo +. I was able to use the DS File app to download the videos to my iPhone (within the DS File app) and then I could play them on my iPhone, but never on the DS Photo + app. I was convinced that there was some sort of bug that hadn't been addressed yet. Then I saw this post, in particular what antoynmo wrote right above you post. I used the Synology Assistant to upload my videos, and voila, I was able to view my videos on my iPhone, through the DS Photo + app. I have not tried it on an iPod or iPad, but I would suspect that it should work just fine.
I was really annoyed that my converted video files (using Freemake Video Converter) wouldn't work, so I tried one more time. However, this time I set the conversion parameters to create a video file with the same settings as the ones that the Synology Assistant used. The Freemake Video Converter will show you some parameters of the files, so I imported one of the files that I created using the Synology Assistant, and created a custom Preset that would convert my .avi files into .mp4 files similar to the one that the Synology Assistant created. I am glad to report that this worked as well. I am assuming that the DS Photo + app wants the video in a very specific format and the Synology Assistant would create that file for you. However, you could try using another converter and setting the parameters of the converted file to be similar to what the Synology Assistant would do. The Freemake Video Converter has some built in settings for various versions of iPhone, iPod, and iPad. Surprisingly, the built in iPhone 4 setting did not work for me. I was able to get a custom one to work and I also was able to get the built in iPhone 1-3GS setting to work.
Hope this helps.