I would just convert your H264 into a supported format and avoid ProRes if you don't have a lot of RAM and drive space since it makes the compressed H.264 files really big for no real gain. The one HUGE thing is you want your H264 to be in I Frame were each frame is an indvidual frame and makes your CPU use more effecient (Hand brake would be constant framerate). (That is why people go to ProRes it is i frame native BUT it makes the file HUGE and your re-compressing your already compressed files). On windows you would use something like DNxHD but again it is a HUGE file from H.264 source. Ratio s to size RAW 1:1 and ProRess would be 1:4 in size and H.264 would be around 1:0.4
You really want an editing format (i frame) and a delivery format. For editing a x264 lossless works when you are starting with a H.264 source. Unless you are on MacOS there are serious issues with encoding anything ProRes with ffmpeg, handbrake or anything else. Also ProRes file sizes are HUGE (Think 10 times) and I can't see this codec being the future for 4K. Here is a good blog post on H264 and slices http://gentlelogic.blogspot.com/2011/11/exploring-h264-part-...
Here is Apple's official policy with ProRes with anything decoding anything other than Apple Products (ProRes is NOT cross-compatible because you can't encode it other than MacOS):
> “Using any unauthorized implementation (like the FFmpeg and derivative implementations) may lead to decoding errors, performance degradation, incompatibility, and instability.” This is why I avoid ProRes and try to use different i frame codecs.
Davinci supports the following formats inside of the H264 container
H.264 8 bit MPEG4/MP4 (This is what I would target with i frame slices for editing AKA every frame is a indvidual picture)
H.264 (Sony XAVCs) QuickTime MP4 (XAVC MIGHT be the future for 4K codec but everyone has their own codecs right now)
So I would use handbrake to just change it to one of those formats. Once you have glitch free playback save your settings and just a one button operation after that. I rarely have a project that I am not converting my source material first anyways and sometimes my whole job on a project is to clean, process and label the video and audio so someone else can edit. Its a pain and it's called an assistant editor.
> second that it had some inexplicable rendering glitches in the output (visibly glitchy and corrupted frames)
Welcome to Codecs they suck right now and have always sucked. I don't use ProRes because I don't rely on Apple standards due to cross compatibility and Apple seems to just make one's life messerable for no reason every blue moon or so. So when you are exporting you actually have to make sure your source is consistant and if it isn't your going to have issues in playback.
When people tell me the editor is not smooth it is 95% of the time the materials codecs. If you don't have consistent frame rate aka 24 frames per minute your computer is decompressing the video while you are edniting and previewing it. Compressed video for playback normally has a variable playback rate to save the file size and it isn't really an issue except while editing video. So you have the consistent frame rate for editing and a variable frame rate for export. So even though you have an export issue it actually is a transcode issue with your source material, normally.