I have always wondered why Amel left this out of the Amel 54 manuals. There is not really anything in the 55 or 50 either.
I suggest using this point (blue circle 1st photo below) and rigging two opposing blocks with 2:1 purchase (one with cam cleats - 2nd photo below), just like your Mizzen Boom Preventer. This leaves the smaller winch available to use.
A pair of these will make your life easy:
Neither the 54 Users nor Owners manuals say anything about the running backstays. I found this little note that memorializes the confidence Amel had of the Bamar furlers: "The jib furling and the staysail (option) system don’t need any maintenance." I am not sure that statement was accurate, and I did not find a similar statement of Reckmann for the 55 or ProFurl for the 50. Things change.
There is no photo of running backstay rigging in the 55 manual and the only mention is, "If your craft is equipped with a staysail (optional), the running backstays must be installed, regardless of the speed."
There is no photo of backstay rigging in the Amel 50 manual and the only mention is, "If your craft is equipped with a staysail (optional), the running backstays must be installed, regardless of the speed. Do not navigate under the genoa only."
Maybe the above helps.