I have a 3D printer, flexible filaments exist, and I would just have to upgrade my print head to print flexible material.

3D printing has one major draw back. It tends to leave void spaces. Like the objects are not waterproof. So it tends to collect debris in those voids. If the part comes in contact with liquids that contains bacteria then the bacteria can get into those void spaces and grow. So now your 3D dildo is going to be septic.

I know for ABS objects (non flexible) you can use acetone slurry or acetone mist to smooth the object and perhaps remove voids. I have built cock rings with my printer and even with an acetone slurry soak of the object it still contains some voids that can collect debris. For flexible material I am not sure if there are options like that.

So, even if you can print a 3D sex toy you need to know the potential issues with the technology. Also, no you cannot print in a vacuum to remove the voids as it would not be able to dissipate the heat properly.

It depends on how it adheres to the plastic. There may be ways to do this. I am not saying you cannot use 3D printers to make this stuff, just be careful and clean the stuff you make more thoroughly.

Another option would be a to get some kind of coating that is like a skin safe rubber that coats the entire object. I don't know what is out there, but with some research you will probably find some kind of medical use compound used to coat internal medical tools/objects. That is similar to what you suggested, but I was thinking something that is somewhat flexible like the stuff they use to make the toys now.