Re: Battery location
If it was me, I'd wire it just like BMW and Volvo do when they have trunk mounted batteries. They don't have all the excess wiring and relays as shown in the schematic, they simply put the battery in the trunk with a longer cable.
I would also not hook my sense wire to the alternator output, as this limits the voltage at that point, rather extend it over to the main power distribution point, in this case the horn relay. This will assure that there is maximum voltage at that point so all your accessories are fully powered.
If you have a permanent magnet starter you don't want to jump the starter as shown as it will cause the starter to become an unregulated generator if it fails to disengage after starting the engine.
"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968