your first scenario is absolutely a better choice. nothing is ever a guarantee, but if you attempt to enter Serbia from Kosovo and the MOST RECENT entry stamp is NOT from Serbia -- they will almost certainly deny you entry. Schengen citizens should be allowed, but again - there are no guarantees.
the crossings throughout the kosovo/serbia border are operating EXACTLY the same as they were before the IBM (slowly and inconsistently).
if trains are what you like, you can take the train as far as Nis - that's the biggest stop for getting a bus into Kosovo (although buses are available to Kosovo from Novi Sad and Belgrade as well.)
hope that helped.
All Schengen citizens (in this case both Luis and Filu) should be able to enter Serbia from Kosovo by showing their national ID. The passport is needed for entering Kosovo, but once crossing to Serbia, show only your ID. You'll have to show your passport too if asked (most probably), but I haven't heard of a single problem nor a person being sent back to enter Serbia from a proper border.
Of course, forgot to mention, that it doesn't apply to other EU citizens, just those of the Schengen area..