(Note that when I say "the first x", it should be taken to imply "the first x I could find", because I may in fact have missed earlier examples, if they had been worded sufficiently vaguely. The search terms I used to find these examples have been provided. The general timeline trends should still be applicable.)
May 19, 2014 (realtime), March 28, 2011 (Mitakihara time)
The Quest begins. We wake up in an alley. We think we see Madoka as Madokami for a moment. Everyone is too busy hashing out a wish to think much of this.
June 1, 2014 (realtime)
The first time that a proposed vote makes a potential connection between seeing Madoka as Madokami for a moment before Sabrina contracted, and Sabrina's 'Brina-ness.
Found using search term "madoka" and going through from earliest to latest posts.
[x] Oh yeah, just before I contracted, for a moment Madoka's eyes looked golden. Don't know if it means anything or if it was just seeing things from bloodloss though.
June 3, 2014 (realtime)
The first mention of us possibly being a wish-created meguca.
Found using search term "created" and going through from earliest to latest posts.
Magical Girls created via wish in a crapsack universe fighting undead magical girls so an evil alien race can fight heat death is the issue!
July 2, 2014 (realtime)
The start of the idea that Madokami may be responsible for us being ROB'd into this timeline.
Found using search term "created"* and going through from earliest to latest posts.
*this may have been a 2am typo but I don't remember now. Search term may have been "madoka".
Weren't there hints that the ROB in question may actually be Madokami?
Aug 5, 2014 (realtime)
Firn writes a semi-canon Omake about What Could Have Been. It seems to carry the metaphysical implication that Madokami may well be out there, existing, doing her thing, possibly being pray-able-to, even if she doesn't come to be in our timeline.
"Am I interrupting anything?" A new voice.
Sabrina turns. Still unchanged, after millenia, after eons, her blue eyes meet a pair of golden ones framed by long, flowing pink hair. She smiles.
"Huh. How are you here, anyway?"
Madoka shrugs sheepishly, a radiant smile lighting up her face. "I will once have erased my own Witch and myself from existence. I may will never have actually done it, but I did."
"Who are you?" Kyuubey asks.
"Kaname Madoka, I'm pleased to meet you," the goddess smiles.
Aug 8, 2014 (realtime), March 31, 2011 (Mitakihara time)
Direct Intervention pt. 47
As of Oriko's capture, we did
not know with certainty the events of last loop- either Madoka's wish, or that she'd wished at all, for that matter.
Aug 8, 2014 (realtime)
The first speculation that last loop ended with a Madokami wish.
Found using search term "madoka" and going through from earliest to latest posts.
One thing I haven't seen brought up is the possibility that what happened last loop was when madoka contracted to become madokami. After all, we did see a flash of madoka with golden eyes in the very first post.
Aug 8, 2014 (realtime)
Speculation building off the post immediately above, that Sabrina was made or brought to help Homura by the Madokami of last loop.
Found using search term "madoka" and going through from earliest to latest posts.
Following on the Oriko trying to contact Madoka, and following up on what Sightedjt said about Madoka's golden eye, does that mean Sabrina was made/brought here by Madoka to help Homura?
Aug 8, 2014 (realtime)
The first speculation that Madoka's wish may have created Sabrina, and that Homura may have some insight about such a thing.
Found going manually through the quest because I hadn't realized the search bar worked like that.
Homura is probably more familiar with us then she is letting on. She did stutter when she said she never saw Sabrina before, which the hidden text pointed out. The first thing she asked when she pulled Sabrina aside to talk was "why are you here" which also kind of stood out to me. For all we know, maybe Madoka's wish in the previous timeline (whatever it was) somehow created Sabrina which is why she is an anomaly and Kyubey commented that she had a strange soul .
Aug 17, 2014 (realtime), March 31, 2011 (Mitakihara time)
Direct Intervention pt. 58
Homura tells us about the events of last loop. This reveals that it was potentially identical to the canon last loop,
except for the Wish Madoka made at the end. This wish, "that everything could be fixed", is revealed to us. The exact phrasing was clarified on July 4, 2017 (realtime), in "A Red Letter Day pt. 28" (see below). Note Homura's tone in this update; in my estimation, it does not appear that Homura is attempting to communicate to us the idea we were created by Madoka's wish; she seems too distressed with the whole conversation for that.
Aug 17, 2014 (realtime)
The first post immediately after Direct Intervention 58, takes the details of last loop and posits the connection explicitly between Sabrina and Madoka's wish. Also posits that Homura likely sees this connection; suggests it is why she is willing to put so much trust in us.
Madoka wishes everything could be fixed.
A new magical girl shows up out of nowhere in the next loop.
A magical girl promising and trying to fix everything.
I'm sure Homura can see the connection. Probably why she's trusting Sabrina so much.
Aug 18, 2014 (realtime), March 31, 2011 (Mitakihara time)
Direct Intervention pt. 59
The first instance in-story where Sabrina speculates that Homura has made the connection between Madoka's wish last loop and Sabrina's appearance. Probably directly prompted by the line of conversation noted immediately above here.
Oct 11, 2014 (realtime)
The first comment putting forth the idea of Madoka being our mom.
Found using search term "mom" and going through from earliest to latest posts.
Also if Sabrina was created from Madoka's wish... does that make Madoka our mom?
Feb 20, 2015 (realtime), April 4, 2011 (Mitakihara time)
Under the Radar pt. 24
The first update where the idea that Homura and Madoka may be considered Sabrina's parents makes it into Sabrina's conscious thoughts.
Found using search term "mom" and going through from earliest to latest posts.
Feb 25, 2015 (realtime)
Firn confirms, with a mostly-canon omake, that Homura
had made the connection between Sabrina's presence and Madoka's wish last loop, as of the time of our interrogation of Oriko. Note that this confirmation comes several months after the chapters containing the events in question.
April 4, 2015 (realtime), April 5, 2011 (Mitakihara time)
Interloper pt. 36
Update wherein Sabrina almost, but
does not blurt out to Homura that she may be Sabrina's mother in a weird sort of way.
July 4, 2017 (realtime), April 10, 2011 (Mitakihara time)
A Red Letter Day pt. 28
The first time we explicitly mention/joke about the idea that Homura and Madoka may be considered Sabrina's parents. Note Homura's behaviour in this section; she does not appear to immediately follow along with our train of thought, which may imply that she had not previously considered our existence in that light before we brought it up. Homura also tells us the exact phrasing of Madoka's wish last loop: "'I Wish that everything could be fixed!'"