So they would charge you the $238 diagnostic fee even if you're having the work done there? So, in addition to the $481 induction cleaning and sparkplugs? If you're getting the work done there, shouldn't they absorb the diagnostics into the price of the fix?
In my experience, you only pay a diagnostic fee when you have something diagnosed at a mechanic, but now actually repaired there. If you get it fixed at that location, they usually waive the diagnostic fee because you're paying them for the work.
And yeah, no more dealership for that car. I'm not sure of the terms on your extended warranty, but in most cases, having receipts from your indy showing that proper maintenance was performed (or even receipts from buying supplies, if you do your own oil changes) is enough for them to honour the warranty.