HRVs or ERVs can be installed just about anywhere, but should be located inside the thermal boundary of the structure (not above the insulation in a ventilated attic.)

Stabilized cellulose (aka "wet spray cellulose") is fine in attics, it's done all the time, and in addition to limiting sag it keeps the material from moving around with attic ventilation air currents in windy locations. (It's also always sulfate-free- you don't even have to specify as such the way you would want to for dry-blown.) The ability to install it at the true 1.3lbs (after drying) nominal density requires a competent & diligent installer though. The lower you go in density the lower it's air-retarder benefit, but it'll still beat most glass or mineral-fiber alternatives on that front. (I haven't seen data on 1lb Spider, but 1.8lbs Spider is pretty comparable to 3lbs minimal dense-pack cellulose on infiltration & convection loss factor. The curves in some of their documents lead me to believe you'd need at least 1.5lb Spider to equal 1.3lbs cellulose at a fixed depth.)

I have no experience with the alleged no-sag ceiling board, but 5/8" goods glued & screwed to furring 16" (or narrower) on center should stay sag-free. At 24" spacing I wouldn't count on any gypsum board to handle it over decades (wet-sprayed or dry blown), but with even a thin OSB backer it would.

Running atmospheric-drafted heating appliances in a tight home requires some planning- you may need to install a means of supplying combustion air for the wood furnace to keep it from back drafting. Even in my modestly-tight home I made sure that the boiler/hot-water equipment is sealed-combustion & forced draft that has no interaction with or reliance upon conditioned-space air. With an HRV or ERV installed there's always at least SOME pressure-neutralizing path to allow atmospheric venting to work, but it's not guaranteed to be enough to keep a big burner like a wood furnace from backdrafting or becoming combustion-air starved. In some cases might cut it just fine until somebody turns on the clothes dryer when the bathroom or kitchen fan is on or something. Testing for backdrafting potential needs to be part of your initial protocol before EVER firing up the wood furnace (or gas fireplace, for that matter, even though the potential hazard is lower with a gas-burner than with a wood-burner.)

Give this a look over and let me know your thoughts: http://www.usg.com/rc/data-submittal-sheets/panels/sheetrock/sheetrock-interior-ceiling-sag-resistant-submittal-WB2195.pdf

Looks like it should handle the load based on their claims.

... according to their claims, yes.

I'd still prefer to see 16" o.c. or less for any gypsum-board though (call me crazy...).

And gluing it to the joists with construction adhesive you go will guarantee you never pop a screw head.

You can add the HRV/ERV ductwork now when the studs are open or go back, cut open several walls & snake the ductwork around & through the finished house, then patch the walls. Its a LOT easier to do it during rough -in. The HRV/ERVs are usually under/around $1000 depending on brand & size. You can compare the effectiveness of the various brands to decide. Venmar is a good brand; Ultimate Air is the highest efficiency, but more $$, Fantech (available from EFI.org) also good. Zender (Zenderamerica.com) is a German machine & has the best ductwork by far (long coil of 3" specialty pipe), but may be more expensive than other units. We use them in place of bath vents, so save some money there. they are worth it!

Please elaborate on bathroom fan setup for this? Basically the ventilator replaces bathroom fans and these just run on timers or as needed?? Is that all you need then is just to have these in the bathrooms? How is it different than a standard bathroom fan.
HRV wise what does everyone think of this unit? http://www.comfortgurus.com/product_info.php/products_id/6967.
Cheapest unit I could find a little over $400, but assume this would install in the basement.

An exhaust outlet ins installed in the bath, with a booster switch which increases the speed of the HRV fan for a certain # of minutes.