upgraded to 2022.2, now get repeated errors "send failed..."

Unfortunately, that isn't the case. Based on my knowledge and experience, if there is power line noise it prevents the PowerLinc from sending anything even though it can send via both the power line and RF. The "dual-band" advantage might help when there is a signal range issue, but if there is network congestion or noise on just the power line then everything comes to a standstill until it is cleared.

Oh my! You've popped Insteon's balloon of deceptive marketing.
For years (nearly a decade), I've believed their hype about "Dual-band" technology which they claim "uses both wireless and your home's powerlines to create a robust network that can easily handle common household interference...". I've always assumed that they had learned about the foibles of powerline line transmission from years of experience with X-10, and had added RF transmission to mitigate the known PL issues (noise from DC-DC converters, signal suckers like surge-protectors, phase-coupling problems, etc). There's a nice little chart on their website that lists the RF channel as "Physical Layer 1", and Powerline as "Physical Layer 2". From that, one might assume that transmitting commands over RF would be the first priority in the modem. But from your comments, it appears that one would be wrong. Thanks for the heads-up.

When I disassembled my 2413 PLM, I was pleasantly surprised to see two antennae for the 915 MHz radio (3" long wire stubs oriented vertically). That's 100% more sophistication than I was expecting! However, if the modem doesn't transmit anything when it detects noise on the power line, it's not clear if the antennae are providing any benefit.

Is there any way to disable the powerline transmission entirely on the PLM and only use radio (assuming every device is dual-band)?

[matt (support)]

Not that I'm aware of, but you might google around and see. Maybe clipping something off the board will disable it?

Is there any way to disable the powerline transmission entirely on the PLM and only use radio (assuming every device is dual-band)?

I don't know if the Insteon API provides a means for the programmer (ie Matt, Jay, et al) to instruct the PLM not to send out power line commands. They might be able to comment.

One notion I had when I was troubleshooting the frequent errors in the Indigo log was to plug my PLM in behind a serious powerline filter, one that would attenuate all powerline signals. I have an Insteon FilterLinc that I use to isolate a UPS (notoriously noisy devices) from its branch circuit. I also have a couple of DIY filters that I made years ago when I started out with X10 gear. In theory, if the PLM was behind such a filter, any powerline noise would get blocked and thus not impede the PLM from transmitting (as described by Matt, above). In addition, any powerline signals transmitted by the PLM would get blocked from passing through the filter onto the 110VAC power circuits. That would effectively leave the RF channel as the only way for the PLM to communicate with Insteon devices distributed around the house.

However, I never did the experiment, so I don't know if this idea has merit or not.