[url]https://forum.atlas-games.com/t/counterspelling/428/9]
Here's a discussion we had on the issue a while back. Not surprisingly, when looking over the old discussion I found that I agreed much more strongly with my own opinions than I agreed with the opinions of others, so I advise you to give my words extra weight. 
I think that unraveling the fabric of (form) would typically work.
In regards to your questions about rego (form spells) and so on, I think that for most spells you'd have to evaluate whether the fast cast defensive spell applies to the spell you are defending against. Rego ignem against cloak of flame? possibly yes, rego mentem against loss of but a moment's memory? probably no.