My opinion about your questions :
1- Universal lens hood do exist, but the are most of the time dedicated to standard or tele lenses, since wide angles, due to their angle of view require hoods calculated for each lens. These universal hoods are generally screw type, so you can use them easily with a 50mm or tele lens, but with wide angles, the risk of vignetting is much too important, adding a filter is just impossible without strong vignetting.
Hoods for trans-standard zoom are not very efficient in general since they must be calculated for the shortest focal to avoid vignetting, hence the poor performance at longer focal setting. One exception I know is the 24-70mm f2.8 L (Model 1), where the lens is extending inside the hood with zooming, a clever system for shading since you have the best coverage for every focal length.
Wide angle hoods performance is all relative, since they must by definition be wide and short, that is in contradiction with efficient shading. As I am mainly working with a tripod, I tend not to trust the hood for perfect shading. Even with the longer ones, I use a piece of cardboard to shade the lens further, in order to limit flare as much as possible.
2- Fact that the Canon hood is fitting on the Tamron is either luck or the sign that Tamron copied the Canon bayonet type mount. Problem is every brand has its own bayonet style for the hood, so finding a different brand than fits your lens is try and error. On the other hand, now you know Canon hoods fit mechanically your lens, you have some wider choice, considering you can find a lot of 3rd party hoods for Canon for cheap (just have a look on eBay).
3-About the availability of dedicated hoods, I would tend to think that in the past lots of people have underestimated the usefulness of hoods, and that hoods were mainly accessories for advanced photographers. It is still common to see people with decent lenses with no hood and a cheap Skylight filter on the front lens. My guess is that some people see them as useless, bulky and expensive.
They are actually expensive, because this is the kind of accessories where brands make a LOT of money (in Europe, Canon ET-73 for the 100mm macro is sold around 40 Euro, costing probably 1 or 2 Euro out of the factory), as you are likely to loose or break it, this is a good source of money.
IMO a hood and NO filter is the best option to preserve IQ and protect your lens, since the hood acts as a bumper and can save your filter thread and limit fingerprints in the same time.
Since you shoot cropped, the EW-83H could be the solution, but still I would advise to try it before you buy it.