I do not know enough about Quintavalle, independently, to be able to make a "decision" on him. I think it would have to be one of those things where, for me, I would have had to see him during his testimony to get a better understanding of whether he is making it up or not. Actually see and hear the way he answered questions (and for that I would have to know Italian!), and see his facial expressions, etc..
I have heard compelling arguments from both sides. It is odd that he would mention it a year later. On the other hand, the timing does match up with when we can deduce they woke up, from the computer and cell phone records. It does seem like there was something going on early in the morning.
And it is very possible that they needed some cleaning supply, maybe it was not bleach....maybe it was something as simple as latex gloves.
If it is true that Amanda went in and came back out with nothing, it is possible that she could have stolen something by putting it in her purse. Was it ever mentioned whether or not Q noticed if she had a purse with her? It would have been, IMO, pretty obvious to her not to check-out as that would leave a permanent record that she had been there and what she had bought. So what other solution is there if you need something but you can't pay for it? Steal it. If it was something small like gloves, she could have put them in her purse, or maybe she even had a large backpack-type thing with her. A student wearing a backpack would not have seemed unusual.
Also, with the different variations on what Q was asked, it is possible that maybe he did not want to deal with the police much, thus he only answered whatever questions they asked of him and didn't offer extra information, or didn't make extra connections, at that time.
It is also possible that the investigators, feeling very certain of RS and Amanda's guilt due to the other evidence, put pressure, either subtle or outright, on Q to "come up" with seeing Amanda. I do account that this might be possible, seeing as I read in the Ryan Ferguson case where the prosecutor visited one witness in jail and after that visit, the witness magically was able to point Ryan out, even though his first account was that he could not make out the two people's faces that he had seen the night of the murder.
So at this point, I just don't know.