My wife and I are avid campers and usually camp throughout all four seasons. While we use our off-road Chalet A-frame camper for longer excursions, we still do a lot of backpacking, kayak camping, and primitive "car" camping as well.
We own five tents and over a dozen sleeping bags and these get us through all the seasons and conditions.
That being said, with the kids almost grown it is usually just the wife and I as of late. Sometimes we want to get away for a fast impromptu weekend trip and do what we call "4Runner camping". We sleep in the back of the 4Runner using two 20" x 72" Therm-A-Rest pads (forget those cheap air mattresses!) and our bags. Luckily we are both short so legrooom isn't much of an issue. I am fairly broad and the width between the wheelwells is sometimes hard to deal with. In cold weather we use a Coleman Golf Cart propane heater that fits in the front cup console. We only fire this up a half hour or so before turning in and for a bit before we crawl out of our bags in the morning.
Ventilation is always an issue with tents and even more so when sleeping in a vehicle. (The average adult gives off approximately 1 - 2 pints of moisture when sleeping). A few years back I found a few manufacturers who offered mesh window screens that are double-sided cases that fit over the front and rear side windows, allowing the window to open any amount while still preventing bugs from coming in. I also fashioned a mesh window screen for the rear window using flexible screen mesh and held in place with velcro strips around the window perimeter.
I have purchased a few canopy/tarp products for use off the tailgate over the years but found that making my own was the best solution. I bought an 8' x 10' heavy duty tarp at Lowes and two 7' telescoping poles, stakes, and tie-down ropes from Campmor. This allows us to have the tailgate open if we desire and provides enough protection for two chairs and one or two small tables. We frequently bring our mountain bikes on trips and the tarp also covers the two swing away hitch-mounted bikes very well. I have toyed with the idea of the Kelty canopy you referenced as well. This canopy offers more configuration options than our tarp but I like the simplicity and speed of set-up with my system.
We also have a DAC Explorer 2 tent attachment that does provide more "open" feeling but nothing in the way of more useable room, although those taller folks may find it more comfortable. I got this for <$100 but seldom use it anymore:
DAC Explorer 2 - Minivan and Sport Utility Vehicle Tent, 20747 | Specialized Shelters | Tents | GEAR | items from Campmor.
In my opinion the SUV/minivan tents are a solution in search of a problem. Thet are all inferior in inclement weather when compared to a good 3-4 season low profile tent. None I have seen even have full coverage rain flys. You have to find a flat clearing that fits both vehicle and tent and when you want to use the vehicle the large tent opening must be closed shut. They are never as water or bug resistant as a good tent. I never saw the use or utility of these. What the advantage of having the vehicle attached to the tent is I will never understand.
Remember that a tent offers little to nothing in the way of warmth (aside form keeping rain, snow, and wind off the inhabitants). For warmth and comfort you must rely on a good pad/bag combination.
Hope this helps!