The problem is that whereas a recorded *.ts file has a *.hmt file generated by the Humax which contains the bookmarks, a streamed / saved *.mp4 file won't have this file, you would have to rename the *.mp4 file to a *.ts file and then generate a *.hmt file for the new *.ts file (See Below). Without this *.hmt file you would have to download the *.mp4 file from the Humax and edit it on a P.C.
Note :- Raydon's AV2HDR-T2 will generate a *.hmt file for you but the *.mp4 file would have to be downloaded to Windows P.C. first (it can't be run on the Humax)
It depends what you mean by "reasonable" - but this is a different discussion from the topic title. The quality of picture you get in any particular amount of bits-per-second encoding very much depends on the encoder available to your video editor's export process. H.264 gets you a similar picture with far fewer bits than a simpler encoder, but also demands far more of the PC to produce. The type of content makes a difference too.
What you do is decide what the ultimate output is targeted for (say, a 4.7GB DVD-R), then sometimes the encoder can be told that's what you want, or you try a few output settings to see which gets you to what you want. Then if the output isn't of sufficient quality for what you want you have to think again.
Broadly speaking, you know you get about 90 minutes of video of "DVD quality" on 4.7GB, so you can take that as a benchmark.