1. looking at the pisen battery i cannot see any screws, so the best way is to peal or cut of the cover just under the plastic top (3-5mm should do you can feel where it meets the actual battery cell)
Then gently "pull" the plastic up out a few mm, now you should see some wire (flat metal plate/wire)

2. im not using a desktop charger, but a charger connected to the mugen cover via the 2mm DC plug. But it should not pose any problems because it is still a standard battery but with a battery on top of it.

3. Yes both battery's are connected together, then the connection to 1x circuit board (plastic top) is made, use small wire here, some thing like power wire from a USB cable (RED/BLACK), this should be enough since it can take 500mA+. i recommend making a couple of slits in the plastic to take the wire out, because space is still scarce here length wise, so try your best to keep everything as "flat" as possible

sorry for still not having made a complete guide yet.