I have less information about share point (only basic info). Microsoft released SharePoint for web developers. Microsoft also said SharePoint has compatibility with other .NET t
SharePoint isn't a replacement for ASP.NET. It's an extension of the ASP.NET platform that simplifies the implementation of several common use cases that are mostly relevant to enterprise websites: document management, knowledge retention, collaboration etc... SharePoint relates to ASP.NET in a similar way that Wordpress relates to PHP: it's a specialized extension rather than an alternative.
Same explanation as in (1).
Use SP if the things you need to develop are in the scope of what SP provides, which is mostly enterprise solutions of one sort or another. Here's a good summary of what SP can do.
Again, it would be faster and easier if you're trying to develop the things that SharePoint is meant for. Also, SP isn't so well-documented, so if you're not familiar with it, you might have a slow start.