Interesting question, and the answer doubtless depends on where you are located. My answer, as a resident of Ontario, Canada (and neither lawyer nor insurance agent, though my profession is risk management):
The framework depends on who is the ultimate entity owning assets, worried about liability, and seeking insurance. If an individual owns a car, up here their insurance does apply if someone else with their permission uses it as a one-off. If that someone, however, borrows it regularly, they need to be listed on the policy as an occasional driver. The premium then reflects this. And in both cases, if the borrower has an accident, the "insurance rating" of the owner is negatively impacted. This does not apply to "commercial use", where a fee is charged for the "borrowing". Or even for driving-for-hire, where the (insured) owner/driver transports others for money.
So an IC where an individual has no objection to their personal vehicle being borrowed, once in a while, and no money changes hands, should face no issue (though their insurance rating may get trashed). But if people borrow the car frequently, especially regularly, or money changes hands on a regular basis to "compensate" the owner (whether it's per year/month, per use, per km), then you may be offside. And while I know situations where people have put a single neighbour on as occasional driver on the car insurance policy, there's probably a maximum number above which the insurance company will balk.
The more the IC is moving into this level of complexity, the more I suspect it will be helpful for the IC itself to become a legal entity that owns the assets and insures its risks. I am not actually familiar with this in the IC context, but I do know instances where
a) groups of cottagers have (actually have been forced to have) a road association, legally constructed (in Ontario) as a "common elements condominium corporation" that owns land (the road) and vehicles/equipment (e.g. a grader/tractor and snowplow to maintain it). The cottagers own shares in the condominium corporation, which itself obtains insurance under a commercial policy. The cost of that insurance, and other costs, are split amongst the owners.
b) extended families own a farm corporation, and again insurance owned by that corporation protects it.
I'd expect this type of framework would also work for your "greenhouse accident" example, though (apart from US issues about health costs/insurance we don't worry about up here) the legal ramifications are likely different based on whether the context is more like a visitor to the property vs an employee injured in the course of their duties.
[All this is my somewhat-informed speculation. Liability is one of those areas where legal advice *in one's own jurisdiction* makes a ton of sense. In particular, if this is relevant to someone reading this on a direct level rather than idle musing about ICs, then seek qualified advice and don't rely on my or other random wisdom on the internet.]