Are the accents on "één" to signify that it means "one orange" and not "an orange"?
I'm not a native Dutch speaker, but I can explain from what I hear. When it is just "een" with out the stress marks, one would pronounce it more like "un".
This is a bit late, but I did ask, and yeah, my friend confirmed that I was mostly right with the pronunciation I gave, but he also said that colloquially, you may hear één be pronounced more like "een".
Mainly from Dutch speakers, but then again, I only hear one talk frequently. Let me actually ask him tonight and I'll try to get back with you on that.
Thanks for your further comments! I also decided to try looking it up on this very useful pronunciation guide, which seems to agree with you:
the indefinite article een ('a') is pronounced with voiceless E. It's also written as " 'n " which shows the pronunciation correctly.
That page also has audio samples of both een and één, which can be found by searching for or scrolling down to "E-long".
The first time you introduce a word like één it would be helpful to ensure that the example also contains een to make it easy to compare the pronounciations. Just a thought.