How absolutely early can a woman undergoing IVF (in vitro fertilization) go to mikvah: is there a limit to the 7 deoraita days or is it even earlier like after five or six days, when she would still be in her deoraita nida days?
Questions like this certainly need discussion with an expert. And please contact the wonderful people at www.yoatzot.org, who will be happy to help you. But for theory's sake:
Rabbi Moshe Feinstein (d. 1986) is of the opinion that there is no issue of having an IVF implanted, or even being artificially inseminated, during the nida period. The nida period only prohibits conventional relations.
His contemporary Yoel Teitelbaum, the Grand Rabbi of Satmar, vehemently disagreed and felt that neither a fertilized zygote (IVF) nor sperm (artificial insemination) could be implanted in a woman who hadn't get gone to the mikvah. (As heard from a Rabbi Rakeffet mp3. Here's Rabbi Breitowitz writing about the related question of donor sperm -- for artificial insemination or IVF -- as it relates to mamzerut, referencing HaMaor 15(9) for Teitelbaum's opinion, and Igros Moshe EH1:71 etc. for Feinstein's opinion otherwise. (It's said that Rabbi Feinstein got a brick through his window and disturbing phone calls from followers of Satmar because of his lenient view.)
Rabbi Yitzchak Breitowitz, who has published on fertility methods in halacha, has observed that you could draw the line between artificial insemination (which is closer to normal relations) and IVF, allowing the latter but not the former during nida days, but interestingly enough Feinstein allows both and Teitelbaum prohibits both.
Let me also clarify, because your question implies some confusion:
Normal period, original Biblical law: She begins her period on Sunday. That's Day 1. Saturday is Day 7. She is absolutely and completely a nida during this time no matter what happens. If she's confirmed that the bleeding is done by Saturday afternoon, she could go to mikvah Saturday night.
Biblical law on abnormal period, effectively what's done today in all cases: She waits five days or until her period ends, whichever is longer, and then counts seven clean days.
This means that going to the mikvah any earlier than 7 days from the start of a period accomplishes absolutely nothing whatsoever, from the perspective of halacha. Not at a rabbinic level, Biblical level, nothing.
Thus: according to Rabbi Feinstein, a woman undergoing IVF never goes to mikvah any earlier than she otherwise would (i.e. at least 12 days from the start of a period).
There are some contemporary rabbis in Israel who seek to accommodate Teitelbaum's opinion on some level, which would raise discussions of going to mikvah between days 7 and 12 solely for the purposes of IVF, but that would be discussed invididually.