So, two issues:

Regarding the prospect of requiring liability insurance for guns -- would it apply only to all new purchases, or should it likewise retroactively apply to guns already in circulation...whether they were manufactured 5 years ago or 50 years ago?

I assume that one's gun license itself (apart from any hypothetical insurance) would need to be renewed after an increment of years has passed? If that's the case, then the licensee would presumably need to show competence before having their license renewed. A fee for renewing one's license is reasonable; slapping excess taxes on top of that licensure renewal, on the other hand, isn't.

I would also argue that, since vehicles are more of a necessity than guns, the need for liability insurance is more necessary for vehicle owners than it would be for gunowners.

To your second point: I do agree with you that a "slippery slope," per se, shouldn't be an absolute reason to rule out potentially-wise ideas. My only point is we have to be mindful of the dangers that certain extremes could pose, when crafting the legislation ahead of time. This can be done through carving out reasonable caveats.

For example, would you support a database for federal gun registration if it included a guarantee to gunowners that they wouldn't be taxed for their adherence to the registry? Or is the ultimate intent behind such a registry to outright DISCOURAGE gun ownership in the first place?