1. Some sources say that yuzu seedlings are not polyembryonic and therefore my seedlings could be some other varity. I also saw some other sources say that yuzu could be polyembryonic and therefore it should not be a problem. What can I expect from these seedlings?
The majority of yuzu seedlings are nucellar (genetic clones of the fruit parent) and, of those, many (maybe about half) are polyembryonic. Even if an occasion yuzu seed was not nucellar, it is still likely to turn out pretty much the same as the parent.
(yuzu genes are pretty homogenous, and self-pollination is more likely than outside pollination, unless cross-pollination was specifically attempted)
You can expect the vast majority to turn out "true to seed".
2. I know that citrus seedlings are not true to their parents, however to what extent are they not true to their parents?
It actually depends on the citrus variety, and whether it was pollinated by another different citrus variety.
Usually hybrid varieties are most likely to produce seed that will turn out different from the parents, if the seed was zygotic. (Although hybrid citrus varieties also tend to be a lot less likely to produce zygotic seeds, meaning most of seeds will be exactly the same as the parent)
3. Can I seperate the seedlings from each other (using root inducing growth hormones)?
Yes, if a seed is polyembryonic you can usually separate out the separate seedlings and they'll grow just fine.