Let me try and answer your questions to the best of my ability, Sebb (and Welcome to Android Forums, BTW
There has been so much written about this it has almost become cliché. Of course there are hundreds of different model phones from major players like Samsung, LG, HTC, ect all the way down to import knock-offs from market stalls in Shenzhen, China. Some call it fragmentation, some call it choice, I look at it as free market products. The problem isn't so much with the device as the expectation. That $99 tablet isn't going to work like a $400 pad from a major manufacturer, but many people expect it to be close. Think of it this way ... you are a wine lover but have heard beer is good too, but you don't want to invest in a top line beer so you buy something cheap and unknown at the bar. You could get lucky and find a gem, but the likelihood is that you'll end up with a bad beer and decide you don't like beer in general. That's human nature. We all do it.
Google Play is designed to restrict app availability to the model phones that meet the minimum spec. If you have a low end phone or one with a small screen you may not see the same set of apps as someone who has a Nexus 4, S3 or One X. I wouldn't worry about it. If there is a set of apps that are make or break for you, then just make sure you get a device that can handle it. The Nexus 4 certainly will handle most everything (I'm probably going to pick up one myself when they are available here
You might be surprised that apps crash more frequently on iOS than they do on Android. (Source: Forbes Magazine Article
) The thing is that when they crash on iOS the user is taken right back tot he main screen with no indication that anything went wrong. Most times the user doesn't realize anything has happened at all. Android, OTOH reports the problem to the user so you can troubleshoot it or report it back to the developer. It's really just a different paradigm.
App optimization ...
Android apps, for the most part are scalable, so it's not really a big issue. Some games and apps that rely heavily on graphics do require a minimum resolution to run but those are the exception, not the norm.
Bad guys are out there. They don't care if you have Android, iOS, Blackberry, Windowsphone or a tin can with a string ... they want your information/money/soul. The majority of exploits are through social engineering and will work on any platform. As for the security apps, there is a good bit of discussion as to their usefulness. Personally I don't use them. They do have a bit of an overhead hit on performance and I only ever get apps from legitimate sources like Play, Amazon, or on occasion, getjar.com.
The Nexus will always get them first because they come directly from Google ... except for the deal Verizon struck with their version of the Galaxy Nexus, but that's another kettle of fish. Most phones expect one major official upgrade throughout their life cycle, possibly 2, but that's the beauty of Android. Even after a phone has passed it's useful life cycle there's a huge development community out there ready to do what the manufacturers or carriers won't. I've got a 4 year old phone running the latest version of Android, and it runs pretty well.
I hope I've been able to answer some of your concerns.