a) is not needed if you already do b) - it may even be disadvantageous.
Also not needed, because you already pin the intermediates.
Indeed a nice idea to also backup an EC one.
Because you maybe also loose access to your backup keys...
(or just create more than one backup leaf key and store each of them - securely - at a different place.
Yes and if you pin the intermediates you can follow these "long term" suggestions as the intermediates won't change in the nearer future.
If you only pin your current leaf cert then yes - theoretically.
However if you really only want to pin leaf certs you may better either only renew them (so you can use the cert pin longer than 90days) and(/or) already pre-generate (but obviously not sign them by LE) the certs you intent to use afterwards, so that you can switch them (and the according HPKP header) one by one without having problems.
(bottom line )
But generally the best suggestion IMHO is: Read about the implications of each way you can do, weight the risks up and decide by yourself what best suites you/your webserver/your visitors. This may depend on many factors.
A good article as a first read about HPKP was written by @ScottHelme:
Guidance on setting up HPKP