Thanks @harjms and @Bry for the suggestions and questions. I’ve probed all the signals to understand the electrical connections between boxes, hot/line, neutral, and travelers but perhaps I’ve got something wrong.
I have Romex (not individual conductors) going into each box. However, the wiring in my home may have passed code but is very unconventional with respect to the lack of neutrals, backfed circuits, branches/repurposing of romex behind the walls, etc. So the romex in box 1 comes from box 2, but uses the white wire in the bundle to route the hot/line from box 2 into box 1 (argh – and not even identified with tape as hot and could have been easily confused for a neutral). box 1 has one other circuit and two switches that DO contain a neutral.
Before I exercise the forum any further, I will get more creative about powering the switch with a different hot/line from box 1 (not the original), and use a neutral in the same box to connect to the blue switch and to the white wire in the bundle going into box 2 – or evaluate blue-switch in box 3 with line, load, and neutral with travelers for red/neutral to the remote aux switches in box 1 and 2. I think that will work. I didn’t originally follow this approach because I thought it might be better to configure as non-neutral and perform less disruption to the original “configuration” of the romex bundles as they were established by the builder 20 years ago.
Also, the wiring site has placeholders for 3-way and 4-way non-neutral wiring diagrams, but only contains a 3-way diagram. I made the assumption that the 4-way version just carries the travelers and hot/line signals to the 2nd aux switch. Resources | Wiring Schematics
I appreciate that Inovelli switches can function without neutrals but prior experience with red switches has been mixed in non-neutral configurations. Never had a problem with neutral configs.
However, if my new approach doesn’t work, I’ll draw everything out and repost here.