Technically, this is just an update of what happened to my device, but this can also be the answer to my question: "Is it safe to charge and use now?"
We took the device to a repair store. They tried to charge the battery, but the battery won't charge. Also, the battery didn't explode when it is charging.
They opened the back of it, and they saw that the battery is still a little bit swollen.
They said that the cause of this is leaving the device for a long period of time, just like what I said in my question:
It was swollen many years ago, so I just left it. Now, I just found it and it is now unenlarged.
They also said that we should replace the battery to use it.
So, to answer my question:
1. Is it safe to charge?
- Probably yes? They charged it, but nothing happened. I just don't know if it will explode if I charge it for a longer period of time.
2. Is it safe to use now?
- Well, this doesn't make any sense as the device doesn't have power for it to run. But, if I already replaced the battery, then yes, it is safe to use.