Unfortunatlly you are wrong, completlly wrong. These are not the last questions you will have. these are the first questions you will have. Seeing as you have been a member for a while, you should know, this is the best comunity for discussion of optical disks, and there will always be someone here to try and help. I know I do and I have no problem sending people to other sites for info, as I know that they will always come back here!!!!! You should ask more questions, cause this is the place to get them answered (or a link to an answer).
I'll try to answer your questions and I'm sure others will add info.
If your drive is reliable for scaning, your results should be similar. If you scan a disk 3 times in the same day, you should get similar results (these are not 2000$ machines for testing, they are 40-60$ drives, but for the price, they do an amazing job testing). do not expect identical results. Expect similar results (and if you do not get similar results in the same day, you might not have a good scanning drive).
If your pif results are similar, but you get eratic pi spikes that do not match with transfer tests, then trust the pif.
Play with your drive and see what it can do as far as scanning. So far as I know, some nec drives do well with scanning and others do not. If you get consistant results scanning the same disk then the drive is scaning well, if not, unfortunatlly you might have a drive that doesn't scan well (if it's consistant with pif but not pi then you have that). I currently use an old liteon 8x drive (previous drives were not reliable scanners, the model I have, ame thing, some scanned well, others didn't). Run several scans and see.
As far as will you get the same scans from the same media at the same speed? Not only does it depend on the scanning quality of the drive, it depends on the media. In most cases, good qualiy media that burns good will continue to burn good. Variations in the scan may mean variations in the burn. If you have a fast system in good condition (software wise), you could get very consistant scans. If your system is limited, it might cause some disks to burn worse.
Rather than trying to figure that all out. Just scan a lot. See how things compare and ask more questions if nessasary (there are plenty of people like me that would me more than happy to help). I know that scanning takes a lot of time and effort, but you only have to do it at first. Once you have scanned a few disks in a pack (If a pack is suspect, or for you, unknown, I usally pick a blank disk from the top of the spindle, one form the middle, and one from the bottom, and if all scan similar, I generally asume that the rest of the spindle will perform similar). So you have a spindle of disks that is another brand but the same media code that worked before? do the same 3 disk test. Want to try a mew media code? Maybe scan a little more, but the 3 disk test still works. There are medias codes that I have used so much of with a particular drive and firmware, that I have not scanned a disk in a year or more, because I know it will burn good. And it always does.
If you find your drive to be a reliable scanner, use it as a measuring stick, scan a lot while you are trying diffrent media codes and difrent brands with the same media codes (at least 3 disks per spindle). I promise you, in no time, it will become clear what medias are good for your drive, ans you will have to do little if any scanning. Take the time at first and it will serve you in the long run.