I would have used a small washer to hook into (sorry I edited my post after I wrote it to include that so you may have missed it). You're correct that you could over pull the dent with a slide hammer if you got carried away. My slide hammer has a lot of weight in it, so just gentle taps would do a lot of work. If you happen to over pull you could tap down with a hammer but you're working the metal a lot, and as you found out the metal is quite thin (when you blew through it with the Mig) so you need to be careful how much you work it.
RE: My bonnet, you have also spend significantly less time on your bonnet than I have on mine and I'm still not happy with mine.