Why not post the question here? Someone might have the answer.

if you're following the back to stock method in xda, you'll be flashing the kdz file. In samsung, this is similar to flasing .tar file using odin. It will overwrite all of your phone's partition including your bootloader, recovery, and system.

1. For charging issue, download and install "charging report" from play store. Using original charger + cable will produce 1200mAh at minimum.
2. Slow boot? If after flashing kdz file, i believed that's normal. But if more than 10 minutes, something is definitely wrong.

Please provide detailed feedback for anyone to assist and provide suggestions.

yes. flashing kdz will restore your phone in original state, which is good if you want to sell it, or send for warranty claim.

Since you have back to stock, root it and install twrp. Gaining custom recovery is always my first priority. After that i would like to suggest you install cloudystock, make sure you wipe everything excluding internal storage.

I tried to recall my difficult moments with my G2:
1. Flashed kdz but failed. Solution: Flash using tot method.
2. Power button not working. Solution: sent for repair, rm50.
3. Unable to exit from download mode. Solution: disconnect battery connector.
4. nandroid restore failed at xx%. Flash new rom also failed. Solution: In TWRP, uncheck "use rm -rf instead of formatting".
5. nandroid restore failed at xx%. Flash new rom also failed (in galaxy note 2). Solution: leave the phone off for 30 minutes, once cooled down no more fail message.

You need to know your limit and border (before surrender and leave it to expert for repair), try & error, patient & creativity.

Based on your explanation regarding unpredictable charging process, if I were you, I will sent it for warranty.

My G2 facing this issue, solved it by changing to G3

I've tried last time, it's not a software issue, so regardless you use whatever ROM, the problem still persist. Mine was like towards intermittent, sometimes ok sometimes not. It's very frustrated when you're in a difficult situation and solely rely on gps navigation.

i believed only certain unit affected.

if you asked me, that's good enough (4 hour SOT with remaining 40%)