There is really no reason why a PVR can't use the data embedded in the broadcasts to determine when a program has finished and the next has started, but from our testing experience, most simply use clock time instead. This explains why we don't often see 100% scores for timer recording or series recording in our tests. Before/after padding is a throw-back to analog TV days that we should now be able to do without.
Editing of recordings seems to be regarded as a niche requirement nowadays, judging by the low incidence of manufacturers supporting it. In the current market, the choice seems to be Panasonic or nothing. An alternative is to get a PVR that allows recordings to be copied without protection to another device, and then use eg editing software on a PC.
Personally, I have a Humax 4Tune set for 3min early start, and 10min late end as the default. If I'm recording from a commercial network, I'll often push the late end out to as much as 90min. The contents of its drive can be browsed from my desktop computer, and I can copy across programs to be edited there.