I don't think anyone on here's going to really be able to answer your biggest question, which I think is "Can you please guarantee me that she is absolutely 100% fine as if this had never happened, and that with another child there is 0% chance of this happening again".
Babies (and kids) are such complicated creatures that don't read the text books. So we can see tiny micropremmies who were as sick as dogs who go on to amaze us all, and yet there are term babies who were absolutely fine, then go on to have all sorts of problems for no obvious reason. But we can still try to partly answer, and hope that helps a bit.
1)Was she harmed by low oxygen levels? Probably not. Why not? Well...she was probably getting worse during that time, so the oxygen levels probably hadn't been as low as that for all that time, but had been drifting lower. And more importantly, baby blood is different to adult blood. They have a different sort of haemoglobin to us, which is designed to grab every last bit of oxygen available and deliver it really efficiently to where it's needed - because they're used to getting second hand oxygen before they're born. So that saturation level isn't much lower than a baby is designed to cope with before they're born. So they tend to do surprisingly well - I've seen quite a lot of cardiac babies do well having had levels nearly that low for days and weeks. I can also think of several non-cardiac babies who had levels probably (like your baby we can't know for sure how long) that low for days, and are now bright active healthy kids.
Were her heart/lungs harmed by the PPHN? Probably not. There can be damage from some of the causes of PPHN (meconium in the lungs for instance) but not usually just by the process. You can get a bit of damage from having to use a ventilator, but for someone ventilated for days rather than weeks you'd expect that to heal up as the lungs grow (although being more likely to have wheezy problems in the first few years is a bit more common having been ventilated for any reason as a baby)
2) Causes? Well...most of the time we know, because it's been obvious (meconium, sepsis etc) But not always. I've known a few babies do it for no really obvious reason, and they had a few things in common: they were section babies, they tended not to have had a really good hearty cry when they were born, to be a bit on the small side, often they were a little bit cold and hadn't fed well so their sugars were a little bit low. Obviuously, I don't know how much of that is true for your DD, but PPHN is what I think of as a "vicious spiral" condition. They're more likely to develop it if they've not inflated their lungs really well (cried lots), or if they're a bit cold, or have low sugars, or if they've got a bit floppy and chin tucked partially blocking their airway, but if they're developing it then those problems get more likely, which makes it worse etc. And much of the treatment is about interrupting that downward spiral and dragging them back up...I don't know if your DD did the classic thing of needing loads and loads and loads doing, and then suddenly really improving? Almost like there's a tipping point.
And like sallysparrow, I've never known a sibling be affected.
3)oops, covered some of this in one. Honestly, we always like to follow up for a couple of years any baby sick enough to be ventilated, to check on their development. Because the more milestones we see coming on time, the more relaxed we get. Noone is ever going to be able to 100% tell you that she's 100% totally the person she would have been. But based on what you've told us, she'd be one of the ones that I would be extremely optimistic would actually be 100%.