1. Over the Rocky Mountains is cheaper?
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Cheaper than piping the water down from Canada. The distance is shorter. There is a slow flat uplift of the high plains to the south pass of western wyoming —which is a low gradually sloping up and down spot in the rockies that frontier people took traveling west in the first half of the 19th century. Once the water gets to south pass —gravity would take it all the way to the gulf of california. For that matter you could put a hundred miles of generators in the downhill slope of the pipeline and generate most of the electricity to pump the water uphill. If not, there are natural gas fields and coal fields all over the place to provide power for pumps.
2. Isn’t more volume readily available for a longer period from Canadian sources than merely relying on the Mississippi’s spring surge?
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This is true but it doesn’t solve the problem of Mississippi flooding which every year costs several billion dollars in army core of engineer and FEMA costs. The perfect solution is to kill two birds with one stone. The perfect solution is to solve the flooding problem and solve the drought problem with the same project. This is actually what Hoover dam in colorado accomplished. It was originally designed to solve the problem of flooding in the lower colorado and only later did it fit into the role of providing power and energy.
3. Would downstream Mississippi users object to lower flow, or (as I suspect) would the diversion volumes seem negligible to them?
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We’re only talking about taking water out of the Mississippi during the flood season from March to June. The people along the Mississipi are not going to complain about not being flooded out.
Taking even 10 feet of water off the top of the Mississippi is still an enormous task. That is one gigantic river. That said, there’s also a gigantic number of places out west that could use the extra water.
Practically:
a) what is the likelihood that this ever gets built?
b) if so, when?
(Actually, I'd really like to siphon the money for Moonbeam's monumentally-absurd choo-choo into any water project, ASAP. His dad oversaw much infrastructure development as governor, so pipsqueak kid wants an enduring legacy, too. "The Moonbeam Express.")