you're not the first mama with these problems! I'm not a dentist, but I've read so much here that I've pretty much acquired assistant status (smilemomma, I ain't going for a real job btw; I'm not very good at all with witnessing actual procedures and especially fear/distress
As far as I know, the white stuff is very safe. It's a resin, I think that's some kind of hard plastic. The sealants are essentially plastic, too. They all last a long time.
Smilemomma is a great proponent of nitrous oxide, because the dosage can be adjusted while administering it, so you only add more while it's necessary, and stop it then, which pretty much eliminates the risk of both over- and underdosage. My own dd had a tooth filled using Versed, an oral sedative, when she was 30 months old. It took the edge off, but she was definitely not quiet. Different kids react different to this stuff, and it's only administered once, so there's a luck factor there. Also, dd was helped on a front tooth, which is relatively non-invasive. If your dentist gives you the choice (mine wouldn't for kids under 3yo), I'd go for the nitrous.
In the archives on top of the topics page, there are sections for sedatives and fillings. You can probably find a lot of good info there. Good luck, calm vibes, and let us know how it goes!