lol I understand perfectly the problem of not knowing what's in your addon folder .... I sometimes use the rather blunt technique of trying to play the mission and seeing what error messages I get ;D
(a) should happen anyway. If you are a mission creator and you include an addon in your mission you should publish the url. If you've lost the readme or something, then at least take the trouble to search the net to find it. If you can't find it, then what hope does the poor player have? There is absolutely no excuse for not including correct urls with your mission.
There remains the problem of the urls getting out of date ... you post a mission somewhere with correct information, but the relevant website closes or something.
(b) simply excludes people who don't have fast connections. When I was on 56k downloading a large addon was a non trivial decision. Potentially having to download it several times is madness. Not to speak of the extra space required on the mission servers, as you mentioned.
(c) has some merits, but I for one see no point in downloading an addon if I'm not going to use it in a mission. And there are several websites offering addons, do they release one pack each? In which case there will be a certain about of duplication.
(d) isn't going to happen, thanks to BIS
The whole thing is a bit of a headache. But (a) and Addons at ease are the way to go. If a mission designer can't even be bothered to tell me where all the parts are for his mission......