I completely understand your concern. I also appreciate you being thorough with safety. You are correct, if something goes wrong the merchant (as the importer) is the one that is liable and ends up paying the fees.
However, to be transparent with you, most merchants do not take these proactive steps. I think they should as it is a huge liability, but usually they just sell the products. I do not agree with that approach, but that is why I think you haven't found a lot of information about this online.
Additionally, from the supplier's perspective, they usually are not as considerate when you're a new merchant. If you were pre-ordering 10,000 units they may, but with a single-unit purchase they may have other priorities. Again, not something I agree with but I think important to be realistic with you.
What I would suggest is going to the Oberlo marketplace. It is within the app and there are three types of suppliers. Aliexpress, Oberlo and Verified Oberlo Suppliers. With any of the two Oberlo suppliers, you can email our support team (firstname.lastname@example.org) and they can directly get the liability forms from them (if available). They are on contract with us and will need to send them.
I think this approach would be your best bet, and certainly a good one to try! Here is some more information about the verified suppliers. I hope this helps!