Trying to see both sides of this - I'm not surprised at the dealers reaction - I'm fairly sure dealers of any brand would be saying the same.
It's done very nearly 6,000 miles which is double the service interval. However I think it more likely that overfilling the oil has caused the problems stated.
I wouldn't despair just yet. Clearly it needs a service and the oil level correcting as part of that.
It is possible that the bike will run fine after that, without any additional work.
I can't knock anyone who wants to do their own servicing. But, a friend who knows his way around bikes or a few evening classes at a local college, might be a good idea first.