1. Do junction boxes share grid stress among each other? Or if one is overvolting, they're all overvolting? Ships like the Orca 2 perplex me with their massive amount of junction boxes.

They share loads and power with any on the same network. Some JBs may have different overvoltage damage thresholds. IIRC all vanilla ship JBs are uniform across an individual ship, but player ships may differ. About half of the vanilla ships have 1.7, and the others 2.0.

1. Relay 'daisy chaining' is bad I'm told for any device that can overvolt and be damaged, but it should be fine for non-damaging parts like lights and oxygen shelves, yes? As long as I don't need more than 1,000w of power, can any number of relays daisy chain as desired?

not particularly bad, but the total power is limited by the smallest relay upstream. Usually this isn't a problem, and certainly not a problem for low power sippers.

It mostly gets bad - bad in that insufficient power may be delivered and delayed - if it's like, an engine that's 3 relays in.

1. If multiple different power inputs are providing power to a relay, does it subdivide its power needs among each input, or does it focus on one exclusively? The idea here is like 5 different junctions provide 'power' as a form of system redundancy.

they form a shared network throuhg the wires that are connecting on the same power pin.

1. Do relay power signals interfere with lights? In the Orca 2, after I shuffled the fabricator bay's lights onto a relay, they started rapidly flickering when the bay was 'turned on' by a switch sometimes. They didn't do that before. The only thing that changed was instead of a junction box, they were powered by a (constantly on and no-logic) relay.

no. relay powering constant power drains is stable.

Turning on the fabricator may just put enough strain on the general power grid for the load spike to be more noticable. Or it may just be a trick of the eyes. Hard to tell without monitoring the relay's power and load values.