It would be prudent to replace the batteries as they are past their prime.
Now what gases are there in the casing? Your advisor is correct Andrew. Nothing too serious.
It seems breakdown of the carbon electrodes plus reaction with internal oxygen and hydrogen produces some ethylene, which is used in agricultural storehouses as it ripens fruit quickly, but mostly its carbon dioxide and carbon monoxide. The latter should not be a problem if released in small quantities in the open air.
If I were unable to replace these batteries and really had to do an activation I would carefully (protective gear on) puncture the inflated cells enclosing membrane. Otherwise taking them to altitude could result in ruptures.
After allowing for deflation I would charge with a smart charger.
The battery capacity will be down due to lost chemicals and cathode/anode area but it will hold charge. Once the outer membrane is breached to release gas other corrosive material may excrete itself with more serious consequences.
EDIT. The lithium metal in the battery will react vigorously with water, releasing hydrogen that will on mixing with air explode when the temperature of the lithium water reaction is high enough. If a short occurs the stored electrical energy is dumped, rapidly raising local temperatures dramatically.
This is free advice with the usual caveats.
Dispose of thoughtfully.