There is an offset for data tracks, too. Drives recognize and correct it automatically. For an audio disc it doesn't really matter if your music is shifted by 1 or 2 hundredth of a second, at least from a pure listening point of view ^^ But non-audio data usually needs to be bit-accurate, so the drive has to adjust to both the factory and the drive's write offset.
And no, there's no overlap for discs with a negative offset. If the disc has a factory write offset of -647, for example, all data and audio tracks are shifted by those -647 sectors. But, as I mentioned before, the data tracks get adjusted properly, while the audio tracks don't, that's why we have to do that manually ^^
That said, it's possible that EAC loses samples at the beginning of the 1st audio track if the combined offset is negative. It's an annoying EAC quirk. You'll have to make sure that your drive's offset "outweighs" the negative factory offset. I have an LG with +667 which I use to dump -647 (a common PSX factory offset) or similar. If you're lucky, you have a Plextor. In this case you can use PlexTools, which doesn't seem to be affected by that problem
As for IsoBuster, it does know where the data track ends, but it appends the pregap of the 2nd track to it. The reason we need to cut that off is because redump handles the gaps differently. IsoBuster appends a track's gap to the end of the previous track (so the Track 2 pregap is put at the end of Track 1), while redump prepends the gap to the track it actually belongs to (pregap, as per cue specification ^^).
Appending the gap to the previous track is kind of a hackish solution to allow people to listen to their audio tracks without having the (mostly silent) pregap at the beginning.
I hope that answers all your questions