Don't own Freya yet, but anyway by the numbers:
[1] Agreed, since the Elac doesn't have ultra-high-power-handling drivers and fair sensitivity as you noted. OTOH, 400wpc isn't absurd overkill if you want to very cleanly drive those inexpensive but acclaimed speakers. It really depends on how the amps and transducers perform at high power levels.
EDIT: Sorry, crap, I forgot that you can *only* go from SE inputs to Balanced outputs with the active tube stage engaged. My apologies, I got too excited about running the tubes exclusively...
[2] and [3] It's not a tube buffer, it's an SS buffer and you needn't worry about anything when using it SE or Bal, but it won't output balanced from SE in the buffer or passive mode, only with the tube stage activated. With multiple outputs connected, there are some precautions on using *both* outputs at once, it's clearly written in the manual. This thing is quite bombproof. Oh, when the LISSTs come out for this beast, then you will have still more options.
[4] Generally not, but yes in the specific case of the Vidar's input. And the reasons have to do with maximizing sound quality per cost. Upgrades to two Vidars will be very common provided the amp has the typically insane Schiit value/cost ratio. But starting with one Vidar really makes sense for most people, and I expect most of them will never feel the need to go to two.