How can they give you any kind of accurate estimate over the phone.
Did you move a phone camera around inside the set so they could see it
and do the troubleshooting measurements for them while on the phone?

You need to PAY to have a Mitsubishi authorized servicer come out and
diagnose the set and give an accurate detailed parts and labor repair
estimate. Then call Mitsubishi to complain and ask them to help. Mitz
knows of the leaking tube issue, if the board needs replaced, they are
most often replacing the entire set at a major trade in discount for
the customer that is way less than the repair would cost. Having ALL
purchase receipts in hand with serial number and a detailed copy of the
repair estimate from an Authorized Mitz servicer will go a long way.

Barring that, you will probably win in small claims court against them
due to the manufacturing defect that occured after the warranty.