i hate to say this but yes, been down that road, the motherboard sent is a refurb or tested working, although may not have been tested 100% and can be faulty.. usually they carry 30-90 days warranty if your current is expired, depends on country you live and other laws governing this .

now the Engineer is just an outsourced tech or some sort of dispatch, basically a parts changer and far from an engineer...

you said and i quote"I've paid a substantial amount of money for this hardware" yet no extended warranty?

Anyway if not already give dell a call make sure they are actually aware the attempt to repair made it worst and that what the tech said is true and that more parts are coming sometimes the repair man lies just to get out of a bad situation,goes for all repair guys,

Hopefully this dont steer you away, i had my fair share of issues with alot of repairs and replacements with repairs and dealings with techs that can figure out a thing, but in the end a year later purchased another laptop with proper full  shenanigan warranty lol and couldnt be happier so far, 1.5 years later