If I invent some earth-shaking (not literally) device, and someone issues a competing patent that adds something that doesn't really change the function of the device all that much, is my original patent still valid with the appropriate judicial back-up?
There is more than one issue in this question. You used the phrase "is my patent still valid?". Technically patent validity refers to the issue of whether or not a granted patent would hold up if scrutinized in a court or USPTO action. Something that comes after you can't change the validity of what was previously issued to you. Your patent gives you the power to stop someone else from making, selling, using, etc. it does give you any particular right to do anything with your invention.
If someone patents an improvement on your invention (whether or not they know anything about you and your invention) then they can stop people from making, selling, using, etc. that improved version. If they made their improvement by adding something to your invention you can probably stop them from making their own invention. If they made it different by removing something or doing it a different way then you probably can't stop them. Keep in mind that a patent is not a license to do something; it is a lIcense to try to stop people from doing something.
In your scenario they add something that doesn't change the function much. If they invented/filed after you then it needs to be non-obvious over yours and everything else in existence at that time. If it is an obvious addition to your idea then there may be an issue regarding the validity of their patent.
Valid does not equal valuable.
Patents are a negative right - they give you the power to exclude but not to use. The power to exclude gets limited by (some) courts which makes this all more of an economic analysis. Can you afford to infringe?
Do companies patent around other companies? All the time.
If you present to a big company, can they patent around my patent? Absolutely.