Battery swelling in lithium ion batteries occurs due to age, not leaving batteries charging. The battery is designed to stop charging between 95-100% and it has a built in processor to do that.
Regarding the power button and leaving the computer turned on, there are multiple reasons someone might want to turn a computer off when they are not actively using it.
I worked for Apple for 12 years, most of that as a liaison between front line hardware service locations and AppleCare engineering. Battery swelling is a normal Li-ion failure mode that is not related to leaving a device connected to its charger. If Apple had any evidence of that they would decline warranty service on swollen batteries based on the system report of how long it was connected to external power.
The Magic Mouse has the charging port on the bottom because the first gen model used AA batteries and not a built in rechargeable battery. When they moved to the rechargeable battery model, they didn’t want to redesign the whole thing so they put the port on the bottom. It’s bad enough design that Tim Cook uses a Logitech.