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You’re over thinking it. At half-three in the morning!
A TA6 can be filled in at any point, most often it’s after an offer is accepted. The other buyer may have backed out before they’d even seen it.
The ‘couldn’t get a mortgage’ line may or may not be true and could mean all manner of things. The bank might have found a CCJ the buyer didn’t know they have, they buyer may have not declared a loan, sometimes buyers don’t even get an agreement before offering, maybe the agent didn’t check last time hence was super keen this time.
Keep working through as diligently as you are and you’ll not miss anything.
On the other point - no need to be there for the survey, indeed many surveyors will insist you’re not.
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