A6: 
Part 1
I (Brian9200T) did a trial and found as follows:-
Rewind to the start of the program within the 2 hour buffer using the rewind key. Press the Record button. You can now fast forward to where you were and continue watching if you wish. You must leave the box tuned to that channel till the program is finished. (This seems to contradict the manual Page GB31 which says “….the recorder will not save what you are currently watching”.)
Part 2
(Source: Post 2136 by clanchief to Barry’s ‘Freeview PVR…’ thread.)
I think that I understand your problem in that you are possibly thinking that the Humax is like a VCR and you can start an instant or manual recording of a program just before it starts or immediately it starts and leave it to record while you watch or record something else on an other channel.
I learned, I think on this forum, that it is not like that. If you start a program recording before the scheduled (not the actual) start time of a program, the Humax will think that you are recording the end of the previous program and stop at the scheduled finish time (not actual finish time) of that program. This means that if a program starts say 30 secs early, as sometimes happens, any instant recording started at that point will stop after 30 seconds and will not even appear in the list of recordings, as recordings of less than a minute seem to be automatically deleted. To be sure that you will record the correct program in an instant recording you need to press the i button before the record button to see what the Humax thinks is being broadcast at that point in time, which may be different from what is actually being broadcast, very close to program start or end times.
In terms of your point about losing the end of programs, whether a program is recorded instantly from the record button or scheduled for later recording from the EPG, the recording will always finish at the scheduled end time of the program not the actual end time of the program. There is no signal sent by DTT broadcast programs to PVRs to indicate when a program is starting or ending. This is why many people on this forum want a " time padding feature" to allow the user to specify how many minutes should be automatically added to the end and/or start of each recording.
If a program is likely to run over time, if you are setting up a recording from the EPG it is fairly simple to add a few minutes onto the end time by pressing the yellow button after selecting a program to be recorded and editing the end time (see page GB51 of the user manual). A simpler alternative is to also record the following program, then if the program runs over time the ending will be recorded at the start of the next program.
If you are making an instant or manual recording the procedure to add extra time does not seem to appear in the user manual. I initially thought that an instant recording would continue, like a VCR, recording until stopped by the user, or disk space ran out. I learned, I think again on this forum, that it is not like that. It stops at the scheduled end time of the current program being recorded. If you wish to add extra time, in case the program runs over, you have to press the record button again and add extra minutes, by pressing the arrow to the right of the OK button.
Part 3
(Source: Post 2138 by tonycollinet to Barry’s ‘Freeview PVR…’ thread.)
Your program is due to start at 7:30, so you press record at 7:28 (don't want to miss the start, right?). Humax thinks you are recording the program before the one you want, and stops at the end of that program (ie at 7:29). Use the I button to ensure that the humax has recognised the program you want has started, or if you want to set it before the start, use a scheduled recording from the epg.