Not necessarily; laying aside the issue of whether the battery is even removable, a lot of devices expect to be able to lean on the battery for temporary spikes in power consumption. I don't specifically know that this affects phones, but I'd be surprised if it didn't
yes, and typically at the very least a protection circuit that needs to be implemented or faked, or whatever.
i replaced a swollen battery on a cheapo hotspot with an 18650, but i had to remove the protection chip from the old battery and wire that up with my battery holder. if i was more industrious/desperate i probably could have used the one in the 18650, but that was more fiddling than i wanted