First I think you're over reacting to the length of the shotgun. You can get extensions that will make it look the way you want too. And you can also get a laser to add onto the end of your extension that looks like part of the extension so that extension that was 2" too short would look almost perfect.
But any long gun used inside is going to be hard to deal with. Sure it won't be as hard but the whole trick to HD is to have your firearm pointed the right direction in the first place. You don't want to be doing a sweep of your house unless you must. Find a safe zone and protect yourself from there. I have a spot where I will be hiding behind several major appliances (freezer, refrigerator, hot water heater, washer and dryer stacked on each other and a block wall with nothing behind it but dirt). There's a spot where I will be able to hide there and point my shotgun out a direction that will be safe to fire it (another block wall on the other side of the house with only dirt behind it). Nothing short of burning down my house is ever going to get me out of my "fort". If they burn you out there's no way to defend against that anyway.
Then there's the fact you can load a LOT of ammo in a long mag tube like the one that would fit under that shotgun. Talk about a riot gun. You'll be able to load 10 rounds in that tube most likely. That's a lot of firing.
It's not what you want. I get that. I have a 20" barrel on my Express. BTW you can order barrels from after market suppliers that are 18.5". I don't think you have that many options. That's the way liberalism is. BTW I don't judge your country "just" on one law either. They have lots of liberal laws I don't like.
The barrels are interchangeable with the 870 models.
That's because the Express models "are" 870's. And they are extremely effective at HD too.