There is no harm in connecting in a parallel battery. It's no different than suddenly having a larger battery.
That's not necessarily true. If the new battery is severely depleted, you'll be suddenly throwing a high load on the alternator while it's running. I don't know
as that would be a problem, but I don't know that it wouldn't, either. Besides: We're not talking about suddenly adding additional load, we're talking about accidentally removing all loads.
Battery switches are very simple devices.
That they are, but...
The chances that it will fail in a way that disables the make-before-break connection and does not completely disable the switch all together is extremely remote.
I've been working with things electrical and electronic for some 40 years. I can tell you from long and varied experience that nothing could be further from the truth. It is not at all uncommon for switches to become intermittant, especially as they age.