This is the right place to ask, yes.
1) It's not explicitly mentioned, but the answer should be yes. Retraining back to the base trait should be fine.
2) By RAW, there is no restriction detailed against doing this, yes, but realistically any GM will rule that the RAI indicates any form of retraining feats should be subject to the same limited list of options that the original feat had, I imagine.
3) Blinded and dazzled conditions would overlap, but not stack. It doesn't extend the duration or double the penalties, no, but if you try to remove blinded or dazzled, you'd have to remove both instances. However, you cannot have "light blindness" twice because that is a racial trait and you cannot have racial traits twice.
4) No, according to the Advanced Race Guide:
"Unless stated otherwise, all racial traits are extraordinary abilities, and each racial trait can only be taken once."5a) There is no definitive ruling on this subject, and "traits" are not the same as "racial traits" (annoyingly enough), but the text explicitly says "any effects" and the list is deliberately broadly construed with "and so on" written into the list. So seeing as you seem to count as that race for any and all purposes, presumably that would qualify you for their alternate racial traits. However, a more narrow interpretation is that you can only replace Half-Orc racial traits if you possess them as part of being a Half-Orc. This is somewhat in "Ask your DM" territory. Still, any alternate racial trait that gives you the Human race's Skilled racial trait seems to be leaving the door open to trade it for anything the Skilled racial trait qualifies for.
5b) Ask your DM. I suppose technically yes, you could trade Low Light Vision into Darkvision (as Half-Elf) into Skilled (as Half-Orc) into the combo of Low Light Vision, Fey Thoughts, and Fey Magic while retaining Light Blindness.