Good news everybody! Thanks to the advice + illustrations, the problem was solved! My P08 is back to normal, after I got my third hand into function: it was the hold open latch that kept the toggle back...
Unfortunately, I'm afraid that the hold open spring might have a defect(or the mag is too old), since I never saw it work as supposed. But I don't mind too much, now that I'm aware of it. This Luger was bought mostly for collection purpose than shooting practice, so I can live with that. I sure know more about it now, and will learn again from other people like you in the future.
Here's what it looks like (picture taken 4 years ago):
http://www.internet.uqam.ca/web/t9235/p08a.jpg
And how far I went back to check out the pieces then:
http://www.internet.uqam.ca/web/t9235/p08b.jpg
If I can make all numbers match now, it will be perfect for me! Only the toggle parts are not like the other ones: the original portion must have been replaced in the past for whatever reason.