This might be a bit too late of a response but I’ve got the Samyang 14mm f/2.8, Nikon 20mm f/1.8, Nikon 24mm f/1.8 and recently picked up the 14-24mm f/2.8.
The reality is that those primes are sharper lenses than the 14-24mm. This is something I’ve noticed via pixel peeping and looking at lens scores and dxomark. The three lenses out perform the 14-24mm for almost the same price. Yes, the Samyang does distort a lot but that’s fixed by the click of the button and is the sharpest 14mm lens on the market. The 14-24mm outperforms the OLD primes (the old f/2.8 primes) but doesn’t surpass the newer 20mm f/1.8 and 24mm f/1.8 primes. Another advantage of the 20mm f/1.8 and 24mm f/1.8 is that they accept round filters and you can hold filters to the lens because there is no built in petal shaped lens hood.
If you’re after a 14mm lens, just pick up the 14mm f/2.8 by Samyang/Rokinon; I paid $300 for mine used. It’s a monster lens and a fraction of the price. I’ve got no regrets buying it and a lot of photographers I’ve shown love it to and have purchased one if they don’t have a lens that wide already.
However, I still decided to pick up the 14-24mm lens and my reason was simple in all honesty. First, it’s one lens versus three. That means that there’s less to carry and less time spent changing lenses. Second, you’re not going to notice the quality difference unless you’re zoomed in and looking at the pixels or printing extremely large.
As for holding the filter, I wouldn’t recommend it with the 14-24mm because of the petal shaped lens hood. If you hold the filter to it, there’s some light leak from the left and right hand sides which will be noticeable in your photos in the form of reflections/glare/ghosting etc. You will probably have to invest some money in the filter holder and filters. I think I’ve spent over $1k on a Nisi system (holder, adapter for 77mm, 10 stop, 6 stop, and two grads).
All the best.