Problems Pumping Up a Presta Valve

First, it's really helpful to have a pump with a guage on it that measures the pressure in the tires.  I do the "full by feel method" when I get a flat on the road, but I'm pretty religious about using the guage when I'm at home.  A newer pump might let you avoid having to have an adapter at all since they typically have both Schrader and Presta connectors, unless you're having to use an extender or crack pipe for certain wheels.

I've never had a pump "pop off" - in fact, I don't think it should, but I might just not be familiar with your specific model, etc.  I have had problems depending on the tube that's being used though.  If the tube is smooth (no grooves or threads), my pump won't engage properly and I catch hell trying to pump it up.  I solved this by only buying tubes that have threads.  Of course, that's for clincher tires....have to find someone smarter if you're using tubular. 

Now that you've purchased a new bike, I'm here telling you to get a new pump...more money.  Sorry!

At least you can get them pretty cheaply:

http://www.nashbar.com/profile.cfm?category=6000106&subcategory=60001075&brand=&sku=7625&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20Floor%20Pumps

Are you sure that you're pump locks in the "L" position?  My floor pump is unlocked when the lever is down and locked when the lever is up.

Why do you need an adaptor?  Can your pump actually handle road tire pressures?  These are significantly higher than mountain or commuter tire pressures.

I tend to agree that your thought on the locked position is wrong.