The problem is subtler than you first imagine, and more difficult to fix. A question asking about (for example) the use of a hardware feature (say the magnets for Smart Covers on an iPad) is nothing to do with iOS, and a tag synonym that puts all iPad tags under iOS would catch a lot of these incorrectly in my view.
If you apply the same logic to the desktop hardware as well as mobile, then anything tagged Macbook should count to a master OSX tag, resulting in OSX being diluted with questions obout magsafe, batteries and trackpad problems etc.
A better solution might be to take a long hard look at our tags and hit them with a very big stick. Go to the main site tags page, and filter on iOS, there are 13 different ones which is overkill to me. I would suggest that as a start a clean up should remove the IOS-5 and IOS5 obvious problems, but we should also consider dropping minor revision ones, IOS-4.2 (and indeed IOS4.2) and IOS-4.3 etc are really not needed down to this level of granularity, and if they are we should add them all, and even question the need to add 4.2.2 etc for consistency.
The OSX tags do not suffer quite the same problem in that there are (largely) only major versions listed, but they have other problems due to the colloqial versions such as Lion as well as the version numbers. Consider the existance of the following tags:
I could ask a question that I could apply each of those tags to if I had a 10.7.2 Lion Server (and I do...), but let's face it most will just pick one, it's (likely) not synonymed properly and as such we have several competing tags all creeping up to the badge tag required count which would likely all exceed it if consolidated.
So, in summary, I don't necessarily agree with point 1, point 2 was answered elsewhere (they are automatic on count), and 3 would be good but won't be a panacea.
While we are talking about improving the tag system, then in addition to your suggestions, I would suggest a way of selecting a tag from a list, and then be able to dynamically filter a list based on whether the chosen tag is a master or synonym as an aid to being able to select a group of tags and investigate their current synonym structure, which is almost impossible except as a manual endeavour at present (you can only type one filter).