batteries?? not sure

cadence & HR simultaneously - yes
go to Settings setup and tell it which parameters to show in each screen position

yes you can change the batteries 2032 I believe.

You can also customise the data fields you want to display. I have current speed/HR/cadence/distance/and ride time all on the same screen

Yes to both. You can set up the display to show whatever data you like, spread across 3 seperate screens.

The battery in the GSC10 can easily be changed. It is a standard CR2032 type battery. See page 27 of the user manual for the Edge500 (available from the Garmin website).
Ditto for the heart rate monitor, same battery - see page 22 of the manual.
The display is infinitely changeable - you can easily change it to have both heart rate and cadence on the same page (max. of 3 pages, max. of 8 data fields on each). But note - if you use 8 data fields per page, the fields get a bit small. In my case I need my reading glasses - not very practical while going flat out.
Page 35 of the manual explains how to do this - press and hold MENU, select Settings> Bike Settings> Data Fields

Can batteries be replaced in sensors by me? = Yes "easy"

Can cadence and heart rate be displayed simultaneously? = Yes ,, you can have 3 pages of Data and up to 8 fields per page, you can choose any Data for any field "very easy to set up"

Yes, but not in one of the main data fields. Your position is shown within the GPS menu.