My local dealer told me that I must do both the 600 mile and 15,000 services with a Ducati dealership in order to keep the warranty valid...
He told you a blatant lie in regards to the first service.
As far as the 15000mi valve check, that is plausible. The thing is, with bikes and vehicles, your negligence or aftermarket part or actions (stunting/racing/etc) must have been the direct cause of the failure...or they can't void your warranty.
In other words, if you never change the oil and your dash stops working, they can't deny the warranty claim on the dash. Or like I did with my Duramax, if after i installed the aftermarket exhaust, EFI Live Tuner, cold-air induction and removed all of the DEF, DPF and emissions stuff....I had a failure with the Transmission, they can deny coverage. But if the Air Conditioner goes out, they can't deny covering that.
If you do not perform the 15000mi valve check, and have a motor problem afterwards, they would have a strong case for denying that coverage.
On the first service, no.