The maximum safe burning speed depends on the rating of the disc. It is always safe to burn at the rated speed. Many 8X T-Y can be burned safely at 12X or sometimes even 16X with no quality loss.
Burning speed has nothing to do with underscores in filenames. Try making sure you have enabled the Joilet filesystem.
If your writer is randomly producing coasters then it might be dying. Though this can be indicative of other problems too. There is no way to tell for sure unless you get some LED blink code or a professional service person examines it.
A failing drive can burn discs that cause playback issues, but then onethousand and one other things can cause this too.
It is possible that VBR elements in videos can cause synch problems, but it depends on what you use to create the video and what you use to play it back.
Out of synch sound can be a result of many things. CBR audio is safest.
If the DVD has a defective burn than anything can happen depending on what kind of error it is and how your playback device handles errors.
In general, it appears that the top disc(s) of any spindle are more likely to have quality issues than any other discs in the spindles. The reasons for this are not known. Therefore I cannot offer a reason why there are specs of dust on the disc.
When you burn a file, the source copy is not touched. Even if the burn failed, the original file would not have been corrupted. Of course the original file might be defective as a result of some other unrelated problem. If the burned file and original file both fail in the same places then it is very likely the file was defective before you burned it.