(1) I don't have lights or speakers on the arch (yet),
That surprises me and could potentially be an issue. I haven't actually messed with any of the arch wiring, but can only imagine that it travels through the stainless arch tubing where it meets a hole in the hull at the starboard attachment point. If there's no arch wiring, there might not be a hole for the wiring. But that seems unlikely. I have to assume there's a hole there where you could pull your solar cables through. You might possibly get better access to the arch hull hole by removing the engine control panel.
2. ... I tried the holding tank route from the starboard side to no avail, but I have not tried access from the port side as you've recommended and will try that route when I return to the boat in a few days.
Could you explain what the issue was with the holding tank route? As shown in my photo, that's definitely where the flexible plastic wire conduit runs. You'll almost certainly need to rig up some leader lines and use one kind of snake or another to get access to those confined areas. You might also want to get one of those relatively inexpensive little borescope/endoscope cameras on amazon or ebay or whatever, to help you see what's going on in there.
And just to clarify, when I mentioned the port side, I was referring to the interior center channel under the access panels under the cushions in the aft berth. You should be able to find the main conduit tube under the port side access panels. I would guess the arch hull hole is on one sider or the other, but probably not both, and also probably on the starboard side where there's at least the lazarrette and engine panel access.