GM and Ford say they can do it (they didn't give a timeline). A vent is not actually that complicated a piece of equipment, except for the electronics (and I don't know where those will be sourced from). It's a servo air pump. Elon even tweeted that the work wasn't very hard. I'm sure if the gov wanted it done, they would force current manufs to share their designs (for which they would likely get a royalty for every unit Tesla/Ford/GM produced).
And, as you so pointed out, if it takes longer than a week to transition (I'm not necessarily arguing that), then they need to start sooner rather than later. Once we are behind this curve, people will simply die from lack of appropriate medical care.
We are already seeing, likely due to the US population's poor health in general, that 40-65 year olds are requiring higher levels of support than expected:
Nearly 40% of those hospitalized with coronavirus are younger than 54: CDC
You're ignoring my point that existing vent manufacturers have already increased production. And the whole FDA approval thing. I guess we will see if we ever see a Ford branded ventilator. Color me skeptical.