Let's assume you buy 50 at OKW, 50 at BWV, and 50 at VWL, and you have a different use year for each (i.e., the points get issued in different months odf the year), then:
1. The 11 month home resort reservation advantage applies only to points for the particular resort. Thus you could reserve 11 months out with the 50 OKW points at OKW but you could not use those BWV or VWL points at 11 months out on an OKW reservation. Thus, for any trip at one resort for which you would want to use all points or more than 50, you would be essentially relegated to using the 7 month window. The 11 month window is important particularly for holiday times, key off-seasons like December, and most other times of year at BWV. Thus, you may want to try a different plan like end up with 150 points at one resort (and then possibly add at others after reaching that total).
2. Yes they will know the month/use year for your points but having 3 different contract use years is not a real problem except for the headache of keeping track of three different contracts and their different banking dates. When you do reserve, you just reserve some nights with one contract, others in the same trip with another, etc. and then they link them together as one reservation. Note you should not confuse your various use years with when you can go (which is any time of year) or when you can call to reserve (11 months for the particular home resort, 7 months otherwise); all that matters is what points will be available at the time of your trip not the time you call to reserve.