Happy new year.
For Question 1, IMHO, I think the burner is 'learning' from previous burns and is slowing the burn to get good results as it can't burn to this media at 4x. This happens with Liteon drives, and I am a Liteon guy, so I am not sure if this is also true for Pioneers, but it seems like it may be the case. The learning theory is also just for each specific media type. Because it burnt at 4x previously tells me that it tried to until it found out it couldn't burn a quality disc at 4x speed. Since it is now burning good at 2x means it is doing its job correctly. This may all you will get with this media, or this batch of media as batches can vary in quality.
Smart-burn may also be affecting this. Smart-burn is a nero thing which monitors a burn and falls back when needed. It is enabled through the 'options' button next to the 'drive selection' dropdown menu just before you select 'burn' in nero express. It may be selectable elsewhere, but I am being lazy in (not)looking. That is an option you could try(de-selecting 'smart-burn'), but it is there for a reason and you risk getting bad burns but it may burn at 4x.
Q2, If you are burning mostly movies, then the files you are burning are most likely NOT defragged if you have been using disc cleanup and defragging often. If you are trying to back up many different files(possibly fragmented) placed all over your HDD, then this may lead to buffer underruns and slower burning speeds. So this is not a problem
Q3 If you are NOT seeing buffer underruns during the 2x burn, then the flow of data is keeping up with the burn speed and that is NOT causing the burner to fall back in burn speed to match the flow-of-data bandwidth. So that is not a problem. However, for the 4x burn, if the buffer is full, but the burn fails at 1% complete, that doesn't really confirm the bandwidth question one way or the other.
So, your next step would be to try a burn with smartburn disabled if you want to attempt that and probably waste a disc. Save that error log, label it accordingly and post it. Try your new(hopefully good) media save the burn(or error:() log and label and post. Watch all burns to check the buffer levels and continue to observe good burning procedures.
A couple of interesting nero tools you can try. 'Nero cd-dvd speed'..in your nero toolkit..put a previously burned disc( try a good one and a bad one) and 'run test'. It should take about 10 minutes and be a smooth curve of a graph. Then, in the same window, go 'extra>Advanced DAE Quality Test>Error test>run test'. This should take about 15 minutes. You can post results. Save all these for one post.
Yes, media makes a BIG difference.