The visa itself is valid for 5 years. You could literally enter the date it was issued (as stated on the visa they send you), or enter the same day it expires.

That's the whole point: you are permitted to enter Thailand on that visa within its issue and expiry date. There's no obligation to enter within X timeframe after it was issued etc.

There's been reports of people exiting Thailand at the 180 day point, going to another country whether that be by land or air, and then literally re entering Thailand the next day.

If it were me, I personally would leave it a few days at least. But that's just me being cautious.

Oh wait; you're not a British passport holder/citizen?

Well I got my DTV from the UK last year and didn't get any of the additional checks you've been asked for, so either you were unlucky or they're being far more stricter now.