"...void the warranty.", obviously an illogical restriction by "someone" in the Nanni food
chain. Its your boat
; my only suggestion would be to kick the decision up the food
chain with a sketch to show how you want to wire your boat
. If "they" won't bend on such a minor, minor point, then ...its your boat
and do what you think you must do to have a functioning, safe, ABYC compliant system.
On the other hand, I would question the validity of their interpretation of the warranty...does it specifically state that you cannot change the configuration of the alternator output? If this would "void the warranty" then methinks they protesteth too much!!!
Need I point out that, after all, you spent the $ to put their product in your boat
so you should be treated intelligently. What you want to do is technically and operationally correct. To be quite frank about it, wiring
the alternator to the starter solenoid without any over current
protection is not in accordance with ABYC E-11.
Then again, its your boat
(A bit of apology: I am a technical guy...I don't usually rail like this but..."void the warranty"...on your boat