A lot of people seem to be telling you that you’re grabby, but yes, the whole thing sounds suspicious as hell, that your DH was so close to these people, didn’t get a single bequest from ANY of them, and four (FOUR?) separate wills have all allegedly gone missing? I believe in legal terms they would say: Pull the other one; it’s got bells on it.
But I also have to say, for what you claim was a money- and inheritance-minded family, they made extraordinarily foolish decisions. Or they put their trust in people they shouldn’t have. When you have that amount of money, you don’t fuck about with making a will. You do it, register it, copy it in triplicate, put it in multiple locations, find someone you can trust to make sure it gets carried out, and on and on, etc. There’s a reason that, according to Money.com, 70% of wealthy families lose their money in two generations and 90% lose it in three generations, and it’s because of people like your PILs (Here’s the link, OP, so you'll see you’re not alone in families doing this to each other: money.com/money/3925308/rich-families-lose-wealth/ ).
I know you’ve said in Scotland you can’t order copies of wills in any easy/direct way, but even if you could, it sounds like instead of just losing the wills, it could be that the “competing wills” that won were whichever didn’t include your DP (probably ones made before he was born or something ridiculous).
Regardless, it sounds as though the ship has sailed, and your PIL are determined not to do what DH’s grandparents and uncles may have wanted, which was to actually pass something on to the next generation. A lesson in saying EVERYTHING in a will and registering copies of it EVERYWHERE because you can’t trust a lot of people to just “do the right thing” without the law, when money is involved. For all we know, PIL promised blind that they would just pass money to DH and there was no need to make things official and “complicate” them. 
If you feel confident that you can somehow prove PIL have obfuscated the true intentions of the deceased, done something legally “hinky” with the wills, and stolen inheritance from DH in the process, then get a legal consultation. Hell, if you can get a free consultation with a solicitor, I might, for closure. It’s ridiculous that this happened with four separate wills and no matter how corrupt the solicitor was, there was no paper trail that could be followed by another dedicated solicitor.
Good luck, OP. I think your PILs pulled a fast one and it sounds like your DH is still in the FOG (read up on fear-obligation-guilt, if you haven’t already). The fact that PIL can inherit that much, make it clear they have no intention of passing it on (fine, their call), but then have the brass neck to try to force money out of you both is unreal. They don’t sound like very nice people if their attitude toward money extends to other things in life, and based on your most recent comment, it sounds like it does. You’re a very wise woman for being LC with them, and hopefully, your DH will join you soon.