Extending a result submitted to a conference before acceptance/rejection notification
Firstly, this question is relevant to your considerations.

In my community it is quite common to publish a (possibly extended/revised) submitted work as a technical report or a pre-print right after, or before the conference submission. The idea is to get a useful reference for future work falling exactly in the period between submission, notification and hopefully publication. ArXiv, or a TR with ISSN is fine for that. 

Having said that, the option 2 is something I myself often resort to. Also to consider with this option, if the publication date of the "generalization" is after the submission deadline, such a rejection would be baseless.

As you yourself note, option 1 leads to a non-self-encapsulated paper and option 3, even though fine and correct, prolongs the period between invention and publication.