AFA have really lowered their requirements in what they consider MISB. It's very frustrating really.
You need to stop thinking about it in terms of RRP. Yes some purchase for flipping purposes however there are a lot who don't. Remember we're collectors.
There is no way of telling now is there? It could have been a returned product for all the customer knows.
Also, if you take pride in having a MISB collection, it's akin to having a loose collection with a KO Mirage, he looks the same, no one really knows it's not unless you investigate closely, so why does it matter (remember we aren't talking about any financial implications here)?
It matters because the collector knows and their collection feels incomplete.
Double tape does not guarantee that it will hold better.
As for your example, it will depend entirely on the collector. some will look past it and purchase, others, like myself wouldn't buy it no matter how much you're asking for it. It simply isn't an MISB any more, the "S" standing for Sealed, not closed after all.
Also correct. There is no way for you to prove it. If you had the item AFA'd prior to the tape popping, then you'd probably have a better chance.
Hope this helps.