The infinity symbol or character is in the Public Domain, however, using trademarked similarities to the basic symbol that are substantially similar would be bad. While I do not claim to be a trademark expert, in the example of the Ard mark, duplicating the + and - symbols within the infinity would probably make it substantially similar. Otherwise, using the public domain infinity or your original modification of it should be fine. From what I've seen, the overt use of a similar symbol is dangerous if the intent is to trick or deceive customers of a similar product - e.g., a round green and white logo on a coffee shop that is not Starbucks.
Just opinion by observation.