Easy, Jack was “European Wastrel” thin — a very common size for young men on that side of the pond.
The Molly Brown character is the kind, fellow outsider who acts as a friendly ear for trapped Rose and a mentor for bewildered Jack. Of course the suit fits. It’s effectively Supernatural Aid from Molly’s Bag of Holding.
I agree that the insurance company owns the necklace as soon as it paid out the claim to Cal’s family. So how does the salvage work? When Bill Paxton finds the necklace would need to return it to the insurance company? Or, is it a case of before the expedition begins he pays insurance companies a fee to have salvage rights and keep what he finds?
If Bill actually salvaged it from a shipwreck then (or his sponsers) would own. The insurance only owns it if Cal owned it when he filed the claim. If he gave it to Rose then she owns it and the insurance company is SOL because Cal’s dead and his estate was closed decades ago.
It’s fiction of course. So they can do whatever the plot demands.
IRL & IMO …
But if Paxton knew that finding the necklace meant turning it over to the insurance company, why would he bother looking for it? Yeah, yeah, he gets bragging rights and cool pix. Not enough IMO.
So either he’d be doing the job as a fee for service salvage where they pay him $X to successfully recover their property and $Y (comparative pittance) if he fails.
OR …
They do as you suggest and sign away their rights at some discount to the necklace’s value, choosing the sure thing small payback versus the unlikely but still possible big score. And leaving Paxton with all the risk: go big or go home. Or more aptly: either come back with his shield or on it.
ETA: or what @alphaboi867 said while I was typing.
It suddenly occurs to me, as you note, it’s the Banacek reason.
He gets, oh, 10% of the value as a recovery fee. Assuming the Heart of the Ocean is equivalent to the Hope Diamond, Brock could net, oh, say, a cool $35mil fee.
Unless it was 10% of the 1912 assessed value, which would be about $18K. Which wouldn’t cover his expenses. Interesting!
No, since Cals claim is fraudulent
Correct.
- he freely gave the necklace to Rose.
Did he give it to her, or did he say he’d give it to her as a wedding present?
Is it?
To the best of his knowledge, it went to the bottom with Rose, since officially she died in the sinking. The insurance claim (if any) in the 'teens was paid on that basis. WE the audience know the stone didn’t end upon the bottom in 1912. No one else does. And no one saw her drop it in 1996.
Cal wasn’t married to Rose yet; for him to file an insurance claim for the necklace he would’ve had to claim he hadn’t given to her yet. And there’s no reason for him not to since Rose is dead and nobody can contradict him. When Rose turns up alive in 1997 with it still in her possession saying Cal gave it to her how do disprove that? Ironically that’s the only part of her story that she can prove.
Rose, knows so if she step forward with it, his claim is fraudulent. She doesnt have to wait until 1997.
Brock talks to two investors (Dave, Barry) on the phone and mentions there are a lot more places they could find the diamond. It was during this phone call that the charcoal picture was uncovered. They had a leased Russian vessel, so I’m pretty sure… given their financing was obviously in the 7-figures… Dave, Barry, and the Russians did their homework and were assured they could withstand any legal challenges if they found the diamond.
I was going to make a Dave Barry joke, but it’s late and I’m getting ready for bed. Maybe tomorrow. 
Did anyone besides me watch the 90 min National Geographic documentary on the Titanic tonight? They used laser scanning to create a full-scale 3D model of the wreck and the debris field around it. The detail was amazing, and an enormous (theater-sized) viewing area had to be built to study the images.
Missed that, but it looked interesting.
I knew about J.P. Morgan, but I hadn’t known some of the other famous people here had Titanic travel plans: Famous figures who had Titanic tickets but didn't make it on board
There was another documentary on TVO (Ontario Public Broadcasting) last night titled Titanic in Color. Also interesting, but I missed the first half because I hadn’t checked the broadcast schedule. 
It’s been a very long time since I saw the movie (I watched it on Christmas Day when it came out), but I didn’t remember this.
Did she assume a fake identity?
Yes, when she got into NYC she said her name was Rose Dawson.