WOW! We haven't has such a popular and short term active thread in a long time! Two pages in less than 12 hours!

Your numbers for the size and weight and wing loading all seem just fine. They are right in the ball park with some of the bigger gliders found on the market. But if you can I'd suggest stretching the span and aspect ratio a bit more to something more like 16 to 18. At the size and weight you're looking at the Reynolds numbers will be high and support this somewhat more sailplane like planform for a higher efficiency.

I like the idea of the twin spar design as well. Even if you go with a vacuum bag style wing a pair of inner span forward and rear carbon or similar spars will provide the sort of structural strength and stiffness you need for the tail booms as well as the inner half of the wings.

As noted already the angle of attack is set by the flying speed. I think you meant to ask about the angle of incidence. Ideally you would want the fuselage to sit in the most direct into the airflow angle as possible for the portion of the flight that the model needs the most efficiency by reducing the drag. To my thinking the mission you're considering is a mostly up then down sort of deal with only a minimal portion of the flight at any given altitude while collecting. Then it's "back to the lab". You'll need to crunch some numbers to determine a valid climb angle and airspeed and from that determine what the wing's lift coefficient will be. For motor battery effiency you'll want to look at climb rates that are between the airframe's minimum sink speed and the best L/D speed to see where the max altitude per watt occurs.

I know that for my free flight models the best altitude comes from climbing at as close to the minimum sink speed as I can manage. But those are rather draggy models and the same may not hold for this case. A part of me says "minimum sink" for the climb speed but another part is yelling that you want to cover more distance in the climb so the best L/D speed would give more hieght per watt. You'll want to look into this aspect with more attention since so much of the flight time will be spent climbing under power.

I'd simply forget about the solar cells. I doubt you'll be in the air long enough for any sort of meaningful solar contribution. The way to tell is to figure out how long your flight mission will be. If the solar cells can supply more watt-hours per ounce than a similar weight increase in lithium motor cells can provide then fine. But if, as I suspect, a similar weight added to the battery pack results in more watt-hours of energy carried aloft then your answer is clear. Pack in the lithium cells and to heck with solar cells for this application.

If you feel guilty about not "going green" then cover the roof and one side of the flight support van in solar cells. Aim the whole lot at the sun at your starting point and let the solar cells charge the battery bank which you then use to charge the flight packs for the plane. By soaking up the solar energy and moving it into some storage batteries you'll reduce the amount of solar energy that heats up the inside of the support van. So that'll reduce the amount of air conditioning needed in the van. At the same time it means less gas burned to keep the charging bank up. A win-win all around.