Thanks for your kind words.
Currently, the MVC decoders are unable to seek, because they do not build an index file. It's a known limitation, and BD3D2MK3D can't directly overcome it. But you wrote "(non 3D)", and therefore I guess that you don't need a MVC decoder. A good AVC decoder should be sufficient. Have you tried
FFMpegSource2 or a similar Source plugin ? Also, if you really want or need to use DGMVCSource or FRIMSource, you can use the SequentialToSeekable function of the
Sub3D avisynth plugin by slavanap. It decodes silently all necessary frames to jump to the requested frame and is therefore very slow (especially if you need to seek backward), but it works well. You may also be interested in the main feature of the Sub3D plugin: it detects the right depth for your subtitles without the help of any 3D plane, and can either hardcode your subs or generate a 3D plane! Try it! It is slow and not always accurate, but slavanap did a wonderful job anyway.