While you can easily melt a foil pan over a hot campfire, yours will be filled with juicy peaches - so assuming you don't over-cook it, you should be safe. 40 minutes directly on the coals might be pushing it though - consider using a gridiron to control the heat by elevating the pan slightly, and stick around to keep an eye on things... Remember, dutch ovens are thick, heavy, and slow to transmit heat, and the recipes reflect this.
I would avoid doubling either the pan or the foil: you'll end up with an air gap between the two layers, which will likely just end up ruining the outer pan. Pay a little extra for the thicker "heavy-duty" foil and you should be fine.
Wet sand between two pans is an interesting idea... If you try it, report back!
My biggest worry here would be burning the peaches before they have a chance to release their juices (no slow warm-up with aluminum - you'll essentially be frying them). If you can elevate the pan, you should be fine; otherwise, consider macerating them first (if you have very ripe, juicy peaches, count your blessings and ignore this suggestion).
Finally, consider that Harbor Freight sells cast-iron dutch ovens for $25. The quality isn't stellar, and it's extra weight to lug around, but you'll probably end up with better food all the same.
The biggest practical differences are going to be:
Nothing sticks to a well-seasoned cast iron pan. Everything sticks to cast aluminum, even the brands that call themselves non-stick. (I know because I've tried a few of them.)
While it's true the cast iron can crack, rust or chip if not properly cared for, aluminum is reactive and scratches easily, and even if you get one of the newer anodized ones, you lose the benefits of that as soon as the surface starts to chip and peel (which it will). Cast iron will last much longer when properly maintained; I've seen some that are decades old and heard about some that are more than a century old.
Aluminum has better conductivity and will, in theory, provide more even heat than cast iron. However, every cast aluminum vessel I've ever used has had major problems with hot spots. Basically, both are poor choices in this respect, except in the oven, where hot spots are essentially a non-issue; if your oven is like mine and the element turns on and off to maintain temperature, you'll get much better results with a cast iron dutch oven because it will hold its temperature while the element is off.
Cast iron also works great with induction cooktops, aluminum doesn't work at all. [Cast] Aluminum really works best on standard metal cooktop stoves, although it's okay for glass as well. I say okay because most glass cooktops tend to have elements that go on and off, so depending on your particular stove, this might lead to uneven heat when using any kind of aluminum.
Cast iron is heat-resistant. You mention the melting point, but it's more complicated than that; make sure you check the specifications on whatever cast aluminum you're thinking of buying, because a lot of it isn't even oven-safe (and if it is, it's only safe up to 400° F or so). The term "dutch oven" can be quite a misnomer for those pieces as they can easily warp or crack at high temperatures. Spun aluminum is obviously much worse but cast aluminum is only marginally better.
On the plus side, cast aluminum is obviously much lighter than cast iron and requires less care. Enameled cast iron is also easy to care for but is obviously much more expensive than both regular cast iron and cast aluminum.
In your case, it would seem that your two primary requirements (caramelizing sugar and roasting meat) are at odds with each other. For roasting, you want a very steady, even heat. For caramelizing sugar you need precise control, you need to be able to reduce the heat very quickly when you hit the melting point. Cast iron would be absolutely terrible for caramelization, but is a far better choice for roasting.
Personally, I use a regular (non-enameled) seasoned cast iron pot as a dutch oven and just use a small light stainless steel saucepan when I need to do something like caramelize sugar. I'd suggest you do the same, unless you're really low on space and genuinely need one piece of cookware to do it all.
Honestly, the only thing that cast aluminum really has going for it is its price. The ease of maintenance is overshadowed by the fact that the pieces don't tend to last that long, and everything else it's good at is handled equally well or better by stainless steel with an aluminum or copper core. The few pieces of aluminum (or cast aluminum) cookware I still own tend to sit at the bottom shelf at the very back and gather dust, but YMMV.
Best Answer
A few things that I would suggest for browning in general, some of which moscafj has already hinted at:
Dry your meat: Any surface moisture has to be evaporated, cooling down the meat and the pot. A dry piece of meat will brown more easily. For grilling, many people use paper towels, but for stews you can also roll the chunks in flour so it later acts as a thickner ... but if you do this make sure to shake off any loose flour before adding it to your pot.
Work in small batches. You want to have a bit of space around each piece of meat so that any moisture given off can quickly evaporate, rather than pooling and cooling off the pan. (if you're not using flour). Generally I try for at least 1cm (about 1/2 an inch) gap between the various chunks when browning meat.
Be patient. Browning takes a little bit of time. If the food sticks, just leave it alone. This is a sign that the proteins are starting to change. Once they're fully browned, they'll release their hold. (this is a useful test for if it's browned in a large pot, as you can't easily see under it ... just wait until you can prod it from the side and see if it releases ... but don't push too hard, as you can end up ripping the crust that's starting to form and leave that stuck to the pot; if this happens, deglaze before your next batch, and save the liquid (to add back to the stew later).
But specifically for a dutch oven:
If you're crunched for time, it's often quicker to brown large batches of meat chunks under the broiler (grill / top heat) of your oven, rather than in a dutch oven, but you need to keep a closer eye on it.