Clearly Iand12, your experience with your vehicle has caused you significant upset and stress and in any owners book, if anyone else including myself had suffered the issues you say you've had, then rightly they'd be upset.
I would however like to redress the balance slightly. The trouble with public forums is, it is easy to air your grievances and the more vociferous it comes across, it can make people get 'twitchy' about a future purchase. In our own case, we bought an almost two year old SZ-T manual car from a Suzuki dealership with only 4700 miles on the clock and we've had the car now for 5 months. Other than a blown tail light and a blown fuse, we've had no other issues with the car other than the fact I've taken a decision off my own back to purchase a new battery because the Stop/Start system wasn't functioning nearly anywhere near as often as it should. The new battery has clearly resolved the issue because the system now functions when it should. I couldn't go back to Suzuki and demand they fit a new battery, because I suspect the car had been stood for weeks at a time before we bought it and it undoubtedly caused an issue with the Suzuki fit battery which didn't really manifest itself until well into the winter season. Plus I didn't want another 'factory fit' Exide battery stuffed in there.
Anyway that aside, I accept there will always be the odd car that might give an owner problems, it happens across all marques, no car manufacturer will ever turn out a 100 percent zero problem run of a particular model. You have clearly been quite unfortunate. I see your location and wonder too just how many Suzuki dealerships you have in your locale? I'm reasonably lucky, within a 30 mile radius of where I live, I have no less than 7 Suzuki dealerships I could go to if I had an issue. In other words, if I lost particular faith in one dealership, I can very easily go to another without causing me too much hardship. I suspect perhaps due to where you live, you may not have had that option? For instance, come service time, I won't be taking our car back to the dealership we bought it from, I'll be taking it to another dealership 10 miles away in the opposite direction. Different dealerships can and in my experience, definitely sometimes do do things a little differently, especially when it comes to customer service.
It's interesting you report that you find the vehicle 'very unstable' at speed. This certainly isn't something we've experienced. Our car is on the M1 almost every day, at speed, well 70mph, and I find the ride perfectly acceptable. As for the MPG reading, well clearly this can be influenced by several factors. Tyre pressures can be one of the main causes of poor MPG. We run ours at ECO pressures, so high, 39 psi in the back and 41 psi in the front. The amount of weight carried in the vehicle, we've actually added weight in that we now carry a space saver wheel. And of course the type of road you're actually using much of the time. Ours is mostly motorway useage, so 50mpg is generally achieveable much of the time when cruising with cruise control on. Incidentally, we've also found that if we 'splash the cash' and put some super unleaded in, the car gets a little more MPG. It's not placebic, the car just runs better and does more to the tank.
So whilst it's disappointing to read of one customer's unacceptable experience, there are plenty more of us that aren't having any problems and our vehicles are serving us well. As I say, I'm just trying to redress the balance slightly as I don't think it's entirely fair to slate a car manufacturer wholesale just because of one owners poor experience.
I wish you well with your 'battle' with Suzuki to take the car back. It's not something car manufacturers do lightly, especially when you've had the car for several months. Clearly if you have evidence of a course of failures over that time, they might come to some sort of arrangement, but whether it is what your hoping for, is another matter.
I'd like to make it clear, I don't work for Suzuki, I'm merely a satisfied customer at present. Our Vitara is the first Suzuki we've ever owned and I'd be more than happy to buy again as things stand.