You might contact the manufacturer and see if you can get a service manual or schematic. Or see if they have a replacement board you can install.

I have never seen a treadmill that would create problems on a 15A circuit with the other devices you mentioned. Is this a short circuit? Meaning, does the fuse or breaker open IMMEDIATELY when the treadmill is plugged in or started? Or is it an overload condition, where the fuse or breaker opens after a period of time when the treadmill is in use? Also, please describe more precisely what you mean by "it blew the circuit on the machine." Most treadmill manuals suggest a surge protector to prevent damage to the electronics and DC motor controls. I once assembled a treadmill, and one of the screws went right through a power cable in one of the vertical supports, creating a dead short. Check this, too. If a hidden cable is damaged, you might be able to repair it, or you can get a new one from the dealer or manufacturer.

Scratching comes from movement. Denting comes from DROPPING something. Since the machine will be PLACED on the floor (no one is going to drop it from an upstairs balcony). Staining is something you may have to deal with. You may not. Yes the feet may have rubber on them...but the small amount is fine. If it does stain a little bit, you can be content in the knowledge that you can always hide it with an area rug AFTER the work out machines are gone. Trust me when I say this, rugs and mats and rubber stuff can cause WAY more problems to a hardwood floor that 4 rubberized feet (per machine). If you really are worried then remove the hardwood (nope, not kidding) and store it. Then install some cheap cork flooring in there and go for the works you want. When you are done with your fitness studio, you remove the cork flooring and reinstall the hardwood. There. No harm no foul. But you don't NEED to do anything but put the machines down and live your life. Wood is AWESOME at being a floor. That's why the world has been doing it for 10,000 years (probably more...but historical records are hard to find on the subject).

A surge protector may not/is likely not going to work, many simply try to clamp/limit any voltage spikes that might appear on the power line. That doesn't stop noise from passing in both directions. Your treadmill is likely using an internal variable frequency or duty cycle power supply to control the motor speed and it is the switching noise from this process that is likely getting back onto the power line and triggering the smoke detectors. Simple surge protectors won't be able to stop this kind of electrical noise from passing through. I think you need a true EMI filter such as this between your treadmill and the incoming power: https://www.galco.com/buy/Qualtek-Electronics/854-03/003?source=googleshopping&utm_source=adwords&utm_campaign=&gclid=Cj0KCQiAlZH_BRCgARIsAAZHSBkx1NY1NwCUy1KJ2qF5oUzpNn-jS_qaiPTiwDtGuTlA8t0t8JYkHZEaAtR4EALw_wcB There are many sources for such a filter. It just needs to be sized to the max current draw of what is being protected, and then encased so the connections aren't exposed.