You are probably aware  that downgrade of /e/ version is relatively safe, I have done it a number of times with testing versions without any losses. When on stable, be sure to stay on stable.
(One thing to be aware of, I guess, is if the failed upgrade caused “harm”, the downgrade might not carry out a “repair”. I think this is an outside chance – just saying so that I avoid appearing to say this course is absolutely without downsides !)
I have no direct experience of /e/ recovery, so will link this
There is no good reason to delete or wipe data at this stage. A wipe of /system will be safe because you intend to fully replace it with your downgrade image. A wipe of Cache + Dalvic/ART Cache is always good practice (only transient data is deleted) and is useful when you are trying to correct an error.
As always, backup important things before attempting repairs !
 Forgive entry level information where it is not required !