I don't think shorting to 3v3 will damage the I2C pins, but there's an easy up/down test: if your LCD still works, the RasPi is fine.
The ~1800 ohm resistance you saw is normal, and in fact necessary. I2C requires a pull-up resistor on the SDA and SCK lines. The devices themselves don't generate signals, they just have 'open drain' mosfets that pull the lines LOW when they need to send data. It's a technique that allows an arbitrary number of devices to use the same bus, and prevents nasty problems that can happen when one device wants to send a HIGH signal while another device wants to send a LOW signal.
Do you have any kind of instrument that can trace dynamic signals.. oscilloscope, logic analyzer, etc? Multimeters are great at measuring fixed signals, but debugging a data connection with one is about 95% luck.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.