After jailbreaking you can update back to the latest firmware version (5.6.1.1), just make sure that
before doing firmware update you install the latest version of "jailbreak hotfix" from
the snapshots thread so that jailbreak can survive firmware update/install. Reboot couple of times after doing the firmware install.
Having jailbreak on your PW1 will have zero effect on "improved epub support" (
if we are talking about using Kindle's stock reader software and not about using KOReader to read epubs??? which is one of the thing that having jailbreak makes possible). The only somewhat major difference that such old Kindle firmware has in regards to books / "personal documents" is that unlike the newer firmware it will not ignore manually sideloaded cover thumbnails for azw3 books with PDOC flag / cdeType so you can still use
ExtractCoverThumbs script