Just one comment.
Normally it's more interesting to produce DHW (=domestic hot water I suppose) then space heating.
With the the surface you have you can probably make all of your hot water in summer and a part of it in winter.In other words you use 100% of the heat produced.
In the case of spaceheating you can use only the heat produced in winter and in summer you have to dump the heat or cover the collector.In general that means you use much less then 50% of the potential.

As a consequence, in case your installation is for DHW only it's smart to orientate the panels that way you have the maximum year round yield.that means an inclination of about 45° directed south.
If you boost your system(increasing the surface) and has to much hot water in summer it 's good to orientate the panels that way that it's suboptimal for summer and optimal for winter.That means a more vertical position of the collectors(60° or more).In that case you can have excess heat in winter that you can use for spaceheating.

Don't know if your way of orientating the collectors is very efficient.I'm afraid the losses on non heated back outweight the gain of the double surface.
Personally I would orientate them south and isolate the back.

I can't answer your question about the thermosiphon.For calculation you need to know things like the diam and length of the tube,the difference in t° between the hot water going in and the cooler water coming out and the difference in height and not sure if someone over here know the math to calculate the resulting flow.Maybe someone tried out a similar setup but in that case you have to give more details.Or just try it out.