"Can I actually suggest Rory Kinnear for garage thief and Ken Stott for my angry Scottish FIL?"
Since you have seen these people in real life and know what they are like we will have to go with your recommendations.
But...
The only time I've ever seen Rory Kinnear was in the Netflix show "The Diplomat" - that's really good btw - and the character he played (the PM) was extremely manipulative and forceful. Although, perhaps garage thief is equally manipulative as well? I just took garage thief for someone really quite different.
That show also had Rufus Sewell in it as well (I'm not letting this go!), I've since learned from you that he's a tad too old to play DH, so how about a minor role for him as one of the solicitors or barristers in the inevitable court case scene?
Ken Stott? There's a name. Although I only really remember him from his Inspector Rebus days about 15-20 years ago. Has he done much since?
When I first read:
"Especially with FIL in tow who is an angry Scot that would have not minced his words with her."
My first thought was Douglas Henshall. The latest thing I've seen him in is the Netflix show "Who is Erin Carter?" where he played the role of angry Scottish father very well indeed. He was also in Season one of Outlander.
But there again, if we're talking Outlander then you really can't ignore Graham McTavish as the ultimate angry Scot.
So, is DH Scottish? And does he have a brother that looks anything like Sam Heughan?
Do the Scots ride to the rescue, just in the nick of time, at the height of the Battle of the Beansprouts? Or are they the ones that start the War of the Winter Squash?