1) If you want to do stuff like this, you should add separate filters and / or write your own filters in procmail. Note that procmail is case insensitve by default, so unless the panel filters specifically enable case sensitivity, I believe 'viagra' and 'Viagra' should do the same thing.
If you write your own procmail filters, you can use extended regular expressions, which will allow you to do more extensive matching of patterns (it's also easier to accidentally do things you don't mean to, which is one reason we don't let you do it from within the panel).
Something like this might work (DISCLAIMER: this is untested and might need some adjustment to actually work. Please see the kbase article on procmail as well as the links at the bottom of that article for more information. Procmail is a powerful tool, but it's not simple. Also, make sure to test any recipe and make sure it's working before sending it to /dev/null).
2) There are a number of reasons that this could be happening. The emails could be base64 encoded, the strings could be actually slightly different than the strings you're filtering based on.... If you come across such an email in the future, save it to its own special folder and contact support - without seeing the actual email, we can't really guess why this might be happening.
On a more general leval, please see the other recent thread about this where I explain that our email filters are NOT designed to be used as content filters. They won't work well in this capacity, as they will probably trash mail you don't want to filter, and won't filter most of the mail you do want to filter. You're looking for a scalpel, and we're giving you a jackhammer.... a jackhammer is effective for drilling holes in the pavement, but not so effective for open heart surgery.
If you want to do content filtering, I would suggest using a more intelligent sort of filter, either server side or client side. The bayesian filters, for example, tend to be very effective in blocking unwanted mail. The beta builds of Mozilla, Apple Mail, and some other email clients have built in filters of this sort. Spamassassin can also be quite effective if setup properly, and there is some information on that as well in the forum archives.