MrsWineasaurus I honestly don't think anyone here thinks you are a bad person. They think you are the good one and your OH is a dick. To be fair, hearing he thinks the dog isn't big enough and that's why he wants rid, is pretty damning evidence. Only he's not here to have the anger aimed at him so it's seeming a bit like it's aimed at you - it's not.
As others have said, you have there a cross of two of the most stubborn, energetic and independent breeds I know. Being cross bred suggest it may have come from non pet lines as workers are often used for cross breeding. That'll make all those traits even worse. You have a dog that is built, body and mind, to tenaciously dig and dog and dog and then use it's mouth and paws to destroy whatever it finds. To never give up. Makes it a brilliant ratter. Makes it a trickier pet 
.
Increasing the walking will help and defo do that if you can but there are other things you can also do to help use up some of the mental energy your dog has.
We have a Jack that loves, loves, loves a tennis ball. Try that when out on a walk - though at this age and with the trouble you are having, I too would be using a long training lead rather than going off leash. They also love chasing small furry things so you can use something like www.amazon.co.uk/gp/product/B075JLB7L9/ref=oh_aui_search_detailpage?psc=1&tag=mumsnetforum-21&ie=UTF8 to take on a walk and make the furry end dance along the ground to encourage a tug game.
This Jack also bloody adores a toy to destroy. Something furry. We often use www.amazon.co.uk/gp/product/B001PT9BBE/ref=oh_aui_search_detailpage?psc=1&tag=mumsnetforum-21&ie=UTF8 which we play tug and encourage her to shake and kill. It takes her about a week to turn it into an empty rag but she keeps on playing with it long after that. It gives her an outlet for her hunt instinct.
Other things that you can do to entertain terriers is bury treasure in a sand pit in the garden. Treats and toys. It gives them a digging outlet.
They are normally pretty savvy at learning tricks. Have a look at clicker training. Once you get the hang of it you suddenly see how much you can teach the dog and it will think all the learning is FUN. Two minutes a few times a day can go a long way to calming a dog down. 'Leave it' might be one of the first things you try . Some basics here: www.thelabradorsite.com/5-steps-to-charging-a-clicker/
In short, it involves you first teaching the dog that a click = treats. Once he knows that for sure you then work with him to help him figure out what makes the clicker work. An example to teach him to go to his bed might go like this:
- Find a quiet space in the house and take your dog, clicker, treats and a mat you use in his bed.
- With your clicker in your hand ready, pop the mat on the floor.
- Dogs are nosey and will at least look at the mat, maybe even go towards it. Any sign of ANY interaction at all (even just a glance in the right direction) earns a click. Click as soon as you see it. Then give a treat.
- Wait patiently. The dog will then try to figure out what caused the click. he may try lots of things first but sooner or later will look at the mat again.
- Click. Treat.
- Wait until it happens again.
It only usually takes a few turns for the dog to figure out that looking at the mat gets the click and therefore the treat. Now you up your requirements.
- Dog looks at mat and you don't click. Inevitably the dog tries looking again. No click. More likely the dog really tries looking, e.g. a more purposeful stare or even a step towards the mat. Click and treat.
- Every time he gets a stage you up your requirements until you are only clicking when he gets all four paws on the bed. Dogs new to the clicker may take a while but honestly, most get this within a few mins.
- When he is absolutely sure that getting on the mat earns the click and treat then you can add the command. As he gets on the bed say "on your bed" (or whatever phrase you want to use). Repeat that several times and before long you should be bale to say "on your bed" before he gets on it, he then gets on it and gets a click and treat.
Repeat everything above but in the right part of the house for his bed. The point of moving elsewhere in the house is that the dog isn't used to seeing his mat there so is more likely to look at it when you start training. It doesn't take anywhere near as long the second time.
You just repeat, repeat, repeat and now your dog knows to get on his bed.
Whatever you do, this dog will need somewhere around 2-3 hours of your time each day (grooming, training, walking, feeding). Only you know if you have that time but obviously it would be much better if you could share the responsibility with your OH. In all honesty, my idea of marriage is not to say "it's now your problem" regardless of the history or who now does or does not want the dog. That doesn't sound anything like a partnership to me at all.