66ST battery replacement

I have a 4 year old 66ST and I suspect the batteries are getting worn out. I want to replace the Garmin battery pack with off the shelf new batteries. I realize that the 'pack' of batteries are designed with the sleeve around them so that the button in the unit is pressed to allow charging while in the unit.
I am planning on trying one of the fixes I've seen online on how to get that charging button to depress even with individual batteries.

So, my questions are:

(a) I'm assuming that the replacement batteries would be 1.2. Since the Garmin pack says 2.4 I assume that is two 1.2 volt batteries together, correct?
(b) If I can find higher mAh batteries, I will probably use higher rated mAh ratings. Will that be ok?
(c) The other rating says 4.8Wh. What does this mean?

Thanks

Hi,

a. yes
b. yes
c. It mean the battery can deliver 4,8 watts in one hour.
Electrical power is expressed in Watts and electricity consumption is expressed in Watt-hours

Do you have a link for the fixes ?