This is not possible I am afraid the panel will only work with the light it is intended for and can only have one panel at a time.
We don't stock that water feature so we're unaway of the specifcations however the Astron Booster is a 12v solar panel so may be compatible with it.
No, the Astron range has been discontinued as the technology has really moved on. These panel are designed just for replacements of our legacy Astron range.
Correct, you'd need a charge controller to manage the power output of the panel.
The Ah of the battery does not matter as this simply refers to the maximum capacity of the battery. The panel in optimum conditions will provide 10watts of power into a 12 v battery per hr. In summer you can expect this panel to produce 65watts per day and in winter 15 watts per day.