1) Microsoft's API's are limited in scope of protection. And no a .NET wrapper does not exist. The IDN mitigation API's are limited to conversions between IDN and ASCII (punycode) and providing some rudimentary checking of mixed script as a security precaution.
2) Something like http://www.casaba.com/products/UCAPI/ will provide more significant detection of string confusability including the presence of confusable characters in IDN. This API also allows for comparison of two strings such as:
AreStringsConfusable(source, target, DetectionMethods)
UCAPI does have a .NET API but it's neither public nor open source.
3) An open source alternative would be ICU at http://www.icu-project.org/ which includes an API also for detecting confusables similar to UCAPI above. However ICU is C, C++, and Java only.