your thought process is good, but for the boat - perhaps too theoretical...
you will have the same voltage present everywhere on the lighting
circuit (whether 12VDC or 110VAC) and each will have the same current capacity - ie sparks will fly...if shorted to ground. Any load on the circuit has the ability, and will if given a chance, to sink every AMP available until the fuse blows or the wire melts, or the fixture fails.
the idea of one size out and a smaller size back...you have neglected the safety aspect of the return MAY have to carry the full load at some point. as far as the wire is concerned...use the same size both ways. BTW your theory is correct, just not practical in the real world.
16 would be a tad small for 7.5 amps, safety, line
length loss, connector loss, etc. 10 would be overkill...besides it is tough to get 10 in the little light fixture.
And one "homerun" or circuit run to the first light, then to the second and so on will do nicely. You may run into issues fusing the same circuit twice. Having hot terminal strips throughout the boat is not a good idea either...every where you break the circuit will be a potential problem, both electrically and corrosion wise. Not to mention that it must be protected from other systems and human contact so that nothing shorts out what is there.
Buy a roll of #14 and #12 stranded for the cabin loads, and a handful of good name brand connectors, some heatshrink and a decent crimp tool..oh, don't forget the fuses or circuit breakers
for higher capacity loads - check one of the better 12 volt bibles/marine wiring books from the library or buy one and work from there.
You are on track to a successful project, but don't caught in the overthink.