Hi,
First things first... all ultrawide lenses can have issues with flare. It's just the nature of a wide lens, especially shooting scenics, that you'll be more likely to have the sun or another bright, specular highlight in the image... and end up with some flare. The Tokina 12-24 actually is one of the better ultra wide angle lenses handling flare. In my opinion, among lenses in this class only the Canon 10-22 is better. The Canon is unusually flare resistant... the Toki 12-24 is merely "very good". Many other lenses are a lot worse than those two.
Below image was a deliberate test of the Tokina 12-24... a worst-case effort to cause as much flare as I could. I even added a circular polarizer (any filter is a no no, shooting into the sun... a polarizer has two layers of glass, so is twice as likely to cause problems... it was a quality, B+W Kaesemann or MRC filter, though).
Yes, there was flare...
It was pretty easily retouched out in Photoshop.
Here's another flare test example, this time with 24-70 like yours, on 5D Mark II (lens at 30mm, f5.6, no filter)....
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The difference between these two images? I moved a little. Nothing else.
My point is that flare is always a possibilty working with challenging lighting, and often is even more likely with a wide lens. The Toki 12-24 is better than most dealing with it. The Canon 10-22 is even better. The Toki 11-16 is a lot more susceptible to flare (part of the price you pay to get f2.8 with an UWA lens). The Sigma 10-20 and a Tamron UWA I tested some years ago really struggled dealing with flare, too. (The Siggy lens has been revised since, and there are now two versions of it... which might be better... I haven't tested them).
As to your lens choices...
1. Keep your 24-70. It's a great lens... Only the new Mark II might be better. Actually the 24-70 is a better portrait zoom on a crop camera, than on a full frame camera. So you are correct that you will likely need a longer lens, too. It really isn't all that big or noticeable... I bet you're just being self-concious when using it.
2. Regarding a wider angle lens...
One thing that might interest you... The Tokina 12-24 actually can be used on full frame. I've tested it on my 5DII and it works just fine up to about 18 or 19mm wide before it starts to vignette. I don't normally use it becuase it shows some more wide angle distortion than the prime lens I usually use, and it tends to have some chromatic aberration in the corners and at the edges on FF... but it works and I wouldn't hesitate to use it in some emergency situation.
Instead I recommend a rather simple solution... Canon EF 20mm f2.8. It's an excellent lens. I actually like it on either crop or full frame. It's currently my widest on full frame. A couple sample shots made with the 20mm on FF...
I am considering adding the Rokinon 14mm (the same lens sells as Samyang, Vivitar, Bower, ProOptic and others). Such a wide lens is something I'd only occasionally use, so I wouldn't want to tie up a lot of money in a 14mm (such as the Canon). The Rokinon 14mm is both manual focus
and manual aperture. Not too big a deal, it's just slower to use than an AF and electronic aperture lens. If I add a 14mm, it will be in addition to the 20mm, which is my go-to lens for wider than 24mm.
Note: With this and other manual aperture lenses, you
can use Av with the Rokinon. Set the aperture on the lens and the camera will meter and adjust the shutter speed as needed. You can use M and stop down manual metering, or set things up with a separate meter. You can't use Tv or P.
Some day I'd like to have the TS-E 17mm, too.... That would be for architecture, though, primarily. I have to line up a paying job or two to justify such an expensive lens, though! I have 24mm and 45m TS-E lenses.
I haven't compared with the 17-40. A lot of people like that lens and a zoom is convenient. But I already had the 20/2.8 and at one time had the 17-35/2.8L (predecessor to the current 16-35... ) but sold it to buy the Toki 12-24 when I was using only crop sensor cameras. I hung onto the 20mm because that prime lens was sharper edge to edge and has less CA than the zoom did. I just can't compare it with the 17-40, not having used it.
I have and use the Canon 28/1.8, too. I particularly like it on a crop camera... it's just not a focal length I use a lot on full frame. What I like about this prime is that it's small... very compact. It's a great lens for street shooting, for example. It's one of the smallest of the USM lenses.
There are two 24mm f2.8 Canons. One is the original EF lens... It's fine optically, but lacks USM and is a little lower build quality than the 28/1.8. The other is the new, very pricey 24/2.8 IS STM (also the 28/2.8 IS STM). I haven't tried either of these new lenses... I just don't need IS on a wide angle... especially not at nearly double the cost and with a slower aperture (in the case of the 28mm). There are also Sigma 28/1.8, 24/1.8 and 20/1.8 lenses. These are older designs and don't have HSM (Sigma's equiv. to Canon's USM). They are also pretty large and heavy. I was never interested in them, so haven't used them. Of course, there also is the Canon 24/1.4L... great lens, but pretty big and pricey, if the extra two stops of light aren't essential to you. Finally, there is the Canon 24mm TS-E. I use one of those (Mark I version), but it's not a lens I'd generally recommend. It's great for architectural photography primarily. It's also an f3.5 aperture lens. Big, heavy and strictly manual focus too. But I find the tilt shift very handy for some things (I also use the 45mm TS-E).
At or around 24mm, I really don't see much to be gained selling off your 24-70 in favor of some prime. Granted, some of those primes are smaller, but the 24-70 pretty much matches them for image quality and already gives you the f2.8 aperture. And the 24-70 is so much more versatile. You'll miss everything from 25mm to 70mm if you for-go the zoom for one of these primes!
50/1.4 is a good lens... I love it on my crop cameras where it's a short portrait tele. On full frame, 50mm focal length bores me and I rarely use it. Not that it's a bad lens and you might like the focal length on FF. It's a wee bit soft wide open... sharpens very nicely by f2.2 and is solid from there on out to its smallest apertures. Still, there's not a lot to be gained over your 24-70 at 50mm.
If you get serious about any of these primes, you can test yourself, to see if you like and would use the focal length, with your zoom. Tape the zoom ring in the particular focal length you're considering, then shoot with it that way for a couple days. See how much you miss the zoom, if you miss it.
3. Regarding a short telephoto...
If it were me, I'd get the 135/2 instead of the 100mm you're considering. The reason is that 100mm is pretty close to the 70mm you've already got. But, I do use an 85mm some of the time, too (and have a 100mm macro lens, which I don't like using for portratiture and I noted macro doesn't interest you).
Incidentally, the 135/2 works very well with Canon's 1.4X teleconverters. That's an easy way to get some extra reach, if it's needed occasionally.
But really, the 100/2 is a fine choice, too... right in between 85mm and 135mm. It can be used with third party teleconverters, I'm sure (don't know what the IQ would be like). It can't be used with the Canon TCs.
Any of these short tele lenses would be great for portraiture and would complement your 24-70 very nicely.
So, if you want to keep it simple, a three lens kit of 20mm, 24-70 and 135/2 is a pretty good way to go. In fact, I'm packing for a trip right now and those three lenses - along with a 5DII, 1.4X TC, 300mm lens, 580EX II and a few other things - are what's going into my backpack.
