Solar panel & battery math verification
Your approach is basically correct, but I question some of your numbers:

How does "1 day of reserve" only require 30% additional capacity, rather than 100%?

You've neglected to take into account the storage efficiency of the battery itself (Wh in does not equal Wh out), which is only about 70% for lead-acid.

For your 275 mA &times; 9 h = 2.5 A-h daily load, you need a 10 A-h battery, which accounts for your 100% functional reserve, plus 100% "battery reserve".

Your charger will need to supply 7 A-h per day, which is the functional capacity (including reserve) divided by the storage efficiency of the battery.

If you have a very good charge controller, it might achieve 90% power efficiency (but possibly much less). 7 A-h &times; 12 V = 84 W-h nominal.
84 W-h / 0.90 efficiency / 4 h (equivalent insolation) = 24 W panel.