Hello Rafael,
For the device that you downgraded, and things were acting badly, I recommend, if you can, factory resetting the device. That will be tricky, though because it appears that the device is arbitrarily rebooting constantly from your description. If you can get it to stay up long enough to press the reset button for >30s, you might be able to get it to reset to factory defaults. It may also be possible to get it to factory default through the API. Downgrading is not a supported use case since the API and the configuration database have to change going forward, and at times, is incompatible with past versions.
I think it should be safe to say that your app should run fine (you’ll need to test it). If your application is working as a custom application using the Node-RED application version of node.js which I think it is, then the primary dependency is on the Node-RED custom application. Currently we have not had to rev the Node-RED custom application yet to run on the different 5.3.3+ versions of mPower. The risk is that if you upgrade to a future mPower version at some point you may it may break and need the Node-RED app updated. Unfortunately, that is hard to predict.
Thank You,
Jeff