I dont think the window will prove that useful. To get max energy
capture you wan to keep the temp rise in the collector lowe. You also
need a lot of area: this makes a giant glass cover prohibitively
expensive and of limited benefit. Pool heaters are not the same as DHW
Re design I like Nick W's giant coiled pipe approach, but I would
suggest a few mods to improve it.
Space the pipe turns out by 0.5 - 1 pipe width. This makes each turn
capture more sun, improving efficiency.
Place a reflective layer under the coil: this works with the point
above. Silvered mylar should work fine.
Place a polythene cover over the whole assy
Use several tubes plumber in parallel rather than one long one, and
fit service valves to each section. This dramatically reduces pump run
cost and makes it runnable long term by eliminating a reliabilty