I've made guitars that show monopole, long and cross dipoles below 100 Hz as well. Those aren't showing here on this redwood top.

I don't know what it's ouput is. I did use a kill-a-watt meter (on the input) and at the highest volume it drew ~75 watts for the loudest frequency. I never use it at full volume. I set it about 1/2 way, although I don't know if it's linear or logarithmic.

I'd imagine it would depend on the type of material you use. I believe Cladni used iron filings, which may be rough enough to stick to sloped surfaces until vibrated off. I've tried several things that bounced right off a flat topped guitar. Thus my preference for course ground pepper. I'm guessing you'd have to ask a violin maker.

It is a bit odd on this particular instrument. I'm pretty sure that this is a result of the way I "squeezed" the X brace so that it is narrower than the standard angle. I also only used one finger brace on each side of the X. I have not seen the node line extend to where the top meets the sides on the fundamental frequency(s). It is usually 12-25 mm in from the sides, although that can very depending on the powder used and the amplitude and/or placement of the driver.The secondary harmonic that divides the top in two longitudinally usually gets much closer to the sides.

I use a frequency generator built by Don Bradley, who is unfortunately no longer with us. If you have a way to amplify sound from your phone to a speaker, check out the luthierlab app. It has a lot of great functionality including a frequency generator.