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This is an interesting problem. Not interesting to you, I'm sure (probably very frustrating), but interesting to me, as it's one with a twist I haven't heard of yet.
That being said, here's what I can offer for info:
Xylene is a solvent, and your friend was likely correct about the "reactivation" (or maybe more accurate - remelting) of the acrylic sealer. However, re-liquefying the sealer did not address the issue of the minerals coming up through the pavers, so when the sealer re-hardened, the problem was still there.
Hindsight being 20/20, applying a sealer over pavers has a desirable consequence of hardening the joint sand, making poly sand not really necessary - a non-polymeric sand would have performed just fine if sealer was being poured over the top of it anyway. But I have my doubts as to whether the poly sand is what caused the problem.
The white film you're seeing is minerals (aka efflouresence) finding their way to the surface of the pavers, then getting trapped there by the sealer. But if the sealer hadn't been applied, you would still see that whitish film - the only difference is, it would wear off over a period of a few months (usually), or a few years (sometimes).
What I'd be more inclined to believe is that there's something at the site causing this problem other than the pavers, sand and sealer. Possibly some minerals in the soils or the crushed stone base that slowly worked their way up, and those last few rainfalls provided the lubrication/bouyancy to push the minerals to the top of the pavers.
You're contractor did right by you, waiting for a year before sealing the project. I used to recommend at least 90 days, for this exact reason - efflouresence sealed into the pavers can be exceedingly difficult to remove. And to help alleviate that problem, we've stopped offering that service altogether, instructing clients in how to apply sealer should they choose to seal their projects.
To the question at hand - "how do we fix this" - I don't have a clean, straightforward answer for you. The xylene can remelt the sealer again, but it would need to be removed from the pavers as much as possible once that was done, to allow the pavers to "breathe" and expel/wear off the minerals. The possible problems I see with that solution are, adding more solvent to the pavers would likely have the effect of pushing the sealer deeper into the pores of the pavers, making removal much harder. Next, complete removal of the sealer seems unlikely.
If there were some sort of acrylic binder that could be spread over the patio after re-applying the xylene, something that would combine with the acrylic in the sealer to make it easy to remove, that would help, but I'm unaware of any chemical that would do that (doesn't mean one doesn't exist; it just means I'm not a chemist).
So unfortunately, I think your best course of action for the next 2 years is no action at all. In time the sealer will wear off under normal foot traffic and allow the pavers to breathe again, and will likely expel the efflouresence. As a preventive measure, I'd avoid sealing the pavers in the future. If the minerals are coming up through the soil (do you have any whitish film on your home's foundation, or on any face brick?), it'll continue to come up for years.
Hope that helps.
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