The -inf values mean “negative infinity”, and imply that the power spectral density in that particular frequency range is zero. Remember, the values in the /muse/elements/*_absolute paths are the log of the power spectral density, and that log(0) = -infinity. If the time domain flatlines (i.e. becomes a DC signal), then the power in the various EEG frequency bands will be zero.
A note: right now MuseIO sets the /muse/elements/*_absolute data to zero when it considers the data to be “bad”. However, it does not currently consider flatlines to be “bad” and so does not set the data to zero in that case. The way in which we treat “bad” data may change in future updates to the SDK to be a little clearer. Stay tuned!
To your second question, you can just use the /muse/elements/alpha,beta,gamma, etc. paths. Those are relative band powers. In the latest SDK, we added absolute band powers and so had to rename the old ones to make the distinction clear.