I offer ((((HUGS)))) on your loss.
Planning something like this during a time of sorrow is a very difficult thing to do. Just remember that you really don't have to be the perfect hostess for something like this. You have other things on your mind. It's understandable.
Would it be a tremendous faux pas to mention "Hors d'oevre served. Cash bar. Valet parking available for $6" on the back of the invitation? Would it be a worse transgression not to?
I don't see anything wrong with mentioning all of the above.
Valet parking and price should be mentioned on the invite.
"Light refreshments will be served" is a good statement. It means, to me, that there will be some food and soft drinks. A dinner will not be served (so no matter what my usual dinner time is, I wouldn't plan on eating a full meal at the gathering.) Plus, with this statement, I wouldn't expect alcohol to be available.
Deetee mentioned not usually having cash on hand so that a cash bar might be a surprise. I agree. Especially if guests see you with a glass of wine. For this reason I think that putting "Cash Bar will be available" on the invite would be appropriate so that guests can come prepared.
I think that any of these statements can be put on an invitation the same way that "Black Tie Only" or whatever dress attire is appropriate.
These comments can be written in a smaller font as a footnote to the actual invitation. Make the reason, time, place and date more prevalent.