1/2" pex for domestic cold main? Just 1 bathroom? You don't NEED 1" ID pex for the loop to the heat exchangers; your short run in 1" nominal would hardly be noticeable to the 26-99. I'm not sure why you're so adamant to use those manifolds, but depending on spacing of the ports you may or may not have heat creep no matter what due to ghost flow. Seeing as you'll need to pull through a mixer for the future loop, if it were me, I'd still pipe the loop with 1" pex from where the Logstor enters the house, to the plate HX (as close to the WH as possible), with a 3 way Sentry to bypass the coil between heat calls, and 1" back to the Logstor return. Use a small circ to move the garage loop with a pair of closely spaced tees between the house coil and the Logstor, and another circ to pull through the mixing valve for the future loop with another pair of closely spaced tees after the garage loop. You need to have either hydraulic separation or flow checks to avoid ghost flow. If the manifold has 4 ports less than 6" apart, you could easily make this work with just one manifold or even both in series with the extras valved off. If they are more than 6" apart, you will need flow checks. Pick your poison, but I'd opt for hydraulic separation personally- flow checks suck