97 seville sls coolant loss

Don't throw any more money or parts at it until you test the coolant for exhaust gases. You can get a test kit at Napa or have it done somewhere.

I had simular systems with 99STS. I had to remove the small hose fitting from the engine, which feeds to the upper part of the expansion tank via the small dia. black hose. The fitting is dual purpose (it is used as a mounting bolt as well.) The weird thing was that it seemed to be clear, as coolant would slowly drickle from hose when removed from the tank. After removing fitting and inspection, the passage was 90% clogged with a combination of corrosion debris and coolant tablet material. I first tried to push the debris out with ice pick, it would'nt budge. Got smart and drilled out passage, reinstalled fitting and hoses,then checked coolant flow at 100%. 30,000 mi. with no coolant issues.

Had the Coolant tested for exhaust gases and all was kosher.
All lines were clear and flowing. Blew out the metal tube that runs along the top og the engine, changed out pump, flushed system did oil change and ran car for 2hrs with no Coolant loss or overheat. Just went for 20mile drive hills and flat and temp steady at halfway point. I'm keeping my fingers crossed that the pump was the source of the problem.
I did see whitish dried collant when I removed the old pump.

Good to read about the outcome of your efforts. I just completed a plug & wire change, Info found at this site helped with removal of coil packs. I belive the metal tube you blew out connects to the fitting that I cleared. I did not have access to compressed air so had to physicly remove blockage. Were any of the blades broken on the water pump? I read that chips can sometimes lodge in small passage ways so a good flush was a good idea. One more thing the stock hose clamps can loose there compression, replace with good quality manuel screw down clamps. Good Luck!

I did see whitish dried coolant when I removed the old pump.

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That was your leak.

Blew out the metal tube that runs along the top of the engine

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That is the purge line. If it was clogged, it will cause it to overheat.