Hey Pim,
the problem is that certbot 1.12 requires Python 3. EPEL ships all certbot plugins this creates a pretty impressive dependency tree. I had two fix/tweak/adapt roughly two dozen packages to get all the required plugins. Unfortunately the project stalled about 4 months ago as I did not have enough free time to finish the transition but most pieces are actually in place already.
If you only need certbot (or certbot-nginx/a few select plugins) and you could try my COPR repo.
However that COPR was only meant for my packaging experiments so no actual deployment testing. The stuff that was built should work but this is really not tested like the usual certbot updates you get via EPEL. Also certbot-apache is not yet available in my copr (missing Python 3 version for augeas).
I have to admit that I missed the problem that certbot 1.11 does not support "--preferred-chain". If you need a fix within the next weeks the best way forward is likely to use a custom virtualenv and install certbot there until EPEL 7 packages are ready. If you only use packages which I already built successfully and the machine is not really critical you could try my COPR and check if everything is working.
I'll try to get the transition in the next weeks but there are a lot of packages involved so I can't promise a quick solution.