I can't help with how long batteries may sit before they are installed and warranty issues, but maybe I can shed some light on some technical issues.
I suspect your hydrometer is designed for SLI (Start-Lights-Ignition) type automobile batteries, which use a different acid concentration than the deep cycle batteries used in golf carts. So the multicolored (Good/Fair/Weak/Bad or whatever) scales may not reflect a true picture of battery health, but should provide a relative comparison between cells.
Also, the Specific Gravity has to be adjusted for the temperature of the electrolyte to get a true reading. Add 4 points (0.004) for each 10°F above 80°F and subtract 4 points of each 10° below 80°F.
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How often do you recharge the batteries? (They should be recharged after each use, no matter how slight.)
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The batteries will only charge to 37.2. (75% SoC)
How are the electrolyte levels?
What are the individual battery voltages?
Are two of the batteries tapped to run 12V accessories?
What charger are you using?
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You might be able to improve your battery pack's storage capacity be repeatedly charging without any discharges, or just minimal discharges.
When I got my cart, the T-105 batteries in my cart were about 18 months old and would only charge to 85% SoC (37.6V) and after working with them for about two months, they would charge to about 98%Soc (38.1V).
No guarantee you'll get the same results, but it is worth giving it a try.