I want to do some local editing on an AC3 stream. My understanding is that frames are independent, so splicing/replacing of frames should be possible if the bitrate and channel configuration match. Is that correct or are there hidden problems? Would the various normalization/gain options pose a problem?

If it is possible, does anyone know a tool that can do the cutting/splicing?

For the editing part, is there a way to extract individual channels so that I can deal with them in an audio editor? And finally, is there a way to encode it back to AC3 from multiple mono streams, without extra silence padding in the ends, and have it perfectly blend in with the rest of the stream?

Yes, frames are independent, even switching bitrate and channel configuration between frames
can be handled by most decoders, although it is not covered by the DVD standard, at least not
within one track.
I don't know of an editing tool on frame level, shouldn't be to hard to write a basic one, since
the frame syntax is described in the ATSc doc (syncword 0X0B77 etc.)
To extract single channels, you need to decode the frame(s). Afterwards you need to reencode,
there is even an open-source solution to do that, besides the several commercial ones.

I should've known... While AC3 is indeed audio, it's much more popular at doom9.org, which I ultimately arrived at while looking for info. I still haven't finished doing what I wanted, but I believe I have all the details and that it should work.

In case anyone is interested, here's a quick list of needed ingredients: BeSplit for splitting, BeSweet/BeLight for AC3 to WAV(s), Aften for WAV to AC3, simple binary file concatenation for splicing, and possibly WaveWizard to create a 6ch WAV from mono files.

I think there may be some reasons to be careful about splicing arbitrary frames, but for editing of an existing stream I suspect it should be okay if done right.

If anyone wants, I could describe the process in more detail.