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I take it there is a feed line and a return line from the manifold in the house to the carriage house? Yes? Then there is no good reason why the pump controlling the circulation in the carriage house can't be in the carriage house. The only thing you would have to watch out for is the pressure drop in the feed line from the house to the carriage house, and thus the inlet to that pump — most pumps should have at least 5 to 10 psig at the inlet when they are running.

All your control wiring for the thermostat and the pump would be in the carriage house. How the boiler would be controlled is another matter — and that depends on what controls the boiler now. It could be a flow switch on the feed or return, for instance, right at the existing house connection.

What's serving the indirect? Sounds like maybe one 26-99 is for the boiler circ and the other is the indirect, then the Alpha does all 3 heating zones? Sure you could run the circ in the carriage house, but how would you signal the boiler to fire?

RIB has a version that works down to .25a

That would work on most PSC circs Im not sure about an ECM on low setting?

for 35 bucks, ill order one one give it a try