Do these extra holes look a whole lot like your transmission lines? Those could be for the oil cooler circuit, which your truck may not have. If that’s the case, you’re fine to leave them open.

There are two likely issues here. First is that the reluctor wheel on the camshaft is either bad or shifted. Think of the sensor as an eye and the reluctor wheel as the instructions. If the instructions are wrong, the eye tells the brain (the PCM) wrong information, and this can keep the engine from starting. If the eye is disconnected, the brain can use information from other sensors to make a good guess, and the engine can run, though maybe not perfectly.

The second possible situation is that you have a stretched timing chain. This is common on GM vehicles as a whole, though I haven’t heard of your engine in particular having that issue. Ultimately it’s the same eye/instructions issue, but if the actual camshaft is out of position, the engine should be running pretty badly even with the sensor unplugged.

Most shops will do a brake inspection for free or at least cheap, but it depends on what exactly they’re planning to do.

Personally I think I’d trust a shop that charges for inspections more than one that doesn’t. If a shop doesn’t charge for an inspection, they’re losing money unless they actually fix something. That’s incentive for them to recommend work that may not actually be required. However, a good brake inspection should take about half an hour, and $150 seems pretty steep for that amount of work.

Find the owner’s manual, it should have a maintenance schedule on it. That should include just about everything considered basic maintenance. If you follow that (including things that are overdue) and fix problems as they come up, it should last a while.

Just my personal opinion, those vans like to have minor things break a lot (at least things that could be fixed), but as far as the essentials go, they’re pretty reliable.

I’ve always used Mobil 1 more out of habit than anything. I won’t tell you it’s any better than the other major brands. Some of the house brands are probably just as good as well.

It’s not an issue if you get the exact same dimensions (including offset). A lot of the time aftermarket wheels end up with slightly different offset than stock (by 2-3mm), which is fine but not ideal. In my opinion the only real risk is the aftermarket wheel you buy being discontinued before you can buy the other 3. This happens a lot, to the point where a lot of people will buy 5, keeping one as a spare.

That’s your transmission fill plug. It looks a lot like a dipstick, but it’s not. They tend to shrink over time and pop out on these transmissions. It’s usually not the end of the world, but I would recommend getting a new one that will fit more securely. Since it’s only ever removed for transmission fluid changes, you can dab a bit of rtv on the sides of it before sticking it back in, it’ll help keep it in place. That’s what I did a year ago and it’s been fine since

You may be able to tow a trailer, but campers are always worse to tow than the numbers indicate. They have a lot of tongue weight and frontal area. You definitely don’t want to tow a camper with a 5. Maybe a very small popup, but I really think you’d be pushing your luck.