I understand your position perfectly. I'm glad that my own collection was purchased (mostly) before 2000, and has no gold - buying now I would only trust reputable dealers for high value items and would insist on a full and itemised invoice.
My guess (not a legal definition!) is that if the sale is concluded, it's tough luck on the seller - the buyer has bought in good faith and cannot be compelled to return the item.
However, if a buyer asks for the coin at the price offered, the seller is within their rights to say the price is a mistake. The buyer cannot compel the seller to sell an item where a clear and obvious mistake has been made.
Define provenance/pedigree? For example, I'm about to buy an 1806 halfpenny from Michael Gouby. I asked him about provenance and he said all he could remember was that he bought it from Spink at a coin fair. So there's no evidence that it was ever in an 'important collection', but frankly the two names of Spink and Michael Gouby gives me confidence in the coin. Would I have paid extra? Perhaps just a little bit, but it's not that important.
I've not heard of any - the first proofs were the 1971 set. I think those were struck in the early 70s at the London Mint who were too busy before 1971 to bother with proofs.
"This time next year, we'll be Ebay millionaires!*)
* To qualify, you must have offered for sale a minimum of 10 different items on eBay in the previous 12 months priced at £1,000,000 or more. Terms and conditions apply.
Footnote:
I've just entered values for George VI and almost across the board those prices have increased. It seems his reign has been undervalued and is now catching up. (Mind you, mostly there are only values for EF and UNC - but to compare, Elizabeth II is static.)