Brief light contact will not harm the peptide quickly, so just refrigerate once the liquid looks clear. Air itself is not a problem, though I prefer to vent the vial instead. No reason to push air in at all. Avoid overthinking the whole thing and simply proceed.
The main focus is on the three primary GLP receptor agonists. Light is widely understood to harm them once mixed, yet the degree of harm remains unclear. Is complete darkness essential, perhaps requiring night vision gear for the task? Could even a minute or two of exposure during mixing cause real problems? Air raises a separate issue, since standard guidance calls for pushing some into the vial via syringe, but air is also said to degrade the peptide. With a vial lasting around a month and multiple draws, would those air additions add up to noticeable harm?