If the redirect to the testing site actually is temporary, this is one of the very few instances I would recommend using a 302 redirect.  This way, the engines will understand that you aren't making this a permanent move and will, hopefully soon, remove the redirect when the staging site goes live.

Definitely do not worry about the disavowing nofollows, unless the domain itself is somehow egregiously offensive to your brand.  Otherwise, you're safe to stick with dofollows.

I've never encountered an official limit to image filenames, and I'm not sure there is any SEO impact (other than just the engines ignoring alot of the filename in their crawl).  Putting the product description in the filename seems VERY unnecessary and I would try to get some rationale around that.  But for the web usage overall, I've never encountered a filename that is too long.  50-70 characters is alot but not prohibitively.