1) If other working chargers show a voltage when poked with a meter then fair assumption to say the one without is broken. Some things however only start showing voltage long term (or at all) when plugged in though, and a sense wire is commonly used for that. Used more for sensitive gear and things with higher voltages/currents/shock risk but I have seen it in a few laptops.
On 2) I am not sure if you misunderstood.
If you are charging at say 10A in this fantasy world then that is 10A going down one wire and back the other. Might get hot, or otherwise require a chunky wire (which will be costly and hard to bend in the end result). Double the wires (and assume no random extra resistances) and now only 5A is going down each wire which means you can use thinner things, just more of them.
3) If your friend's charger works and the battery lasts an acceptable amount of time for your needs then it is probably OK. You can check if you want but unless it has gone puffy or something then you probably won't gain much.
On 2) I am not sure if you misunderstood.
If you are charging at say 10A in this fantasy world then that is 10A going down one wire and back the other. Might get hot, or otherwise require a chunky wire (which will be costly and hard to bend in the end result). Double the wires (and assume no random extra resistances) and now only 5A is going down each wire which means you can use thinner things, just more of them.
3) If your friend's charger works and the battery lasts an acceptable amount of time for your needs then it is probably OK. You can check if you want but unless it has gone puffy or something then you probably won't gain much.
