Sol... Owned.
I developed a method to get the starting seven underground even in the toastiest of embarks, no reclaim necessary:
Embark: Make everyone a miner, with a military skill at least novice, and give them a copper or bronze pick. Whether you bring animals or not is up to you. They usually seem to burn to death in about 5 seconds. Larger animals do better.
On arrival: With the game paused, put everyone in a "Naked Time" squad. Burning clothes is more dangerous than the sun. Station the squad, unpause for a split second, and pause again. Everyone should be naked. Disband the squad. Now get them underground and out of the raging inferno!!! The best way to do this is to just have everyone mine. Though if you really don't want to bring 7 picks, you could keep the non-miners in the squad and station them underground the moment the miners have opened it. Diagonals abuse to stop smoke seems to help.
If everyone gets undercover promptly, no-one should die, except the animals, who die because they're slackers. Once the fires burn out, collect any surviving embark items, such as anvil, battle axes. Now BURN YOUR BRIDGES! No-one leaves! Remove the upstairs to the surface, or wall off the entrance. Dwarves WILL pop outside at the first opportunity to get a quick tan, at least until there are plenty of more exciting things to do under the ground. So keep them busy with mining until the great outdoors are safely inaccessible. You can re-open the outdoors later when there are plenty of safe distractions.
Using this method, everyone should survive, leaving only the usual occupational hazards in a dwarvern fortress.
With immigration waves, the "NAKED STAT!" thing needs to be repeated. This is quite annoying because you can't actually see the immigrants under the clouds of smoke, but just check every few seconds and add them to a stripper squad which is stationed safely undercover. Also (most of) the immigrants will get upset about being drafted. Arguably they'd be more upset about being "topped and tailed" by their burning cap and shoes. I dug tunnels out to multiple points around the edges of the map. I then marked the entire outdoors area low traffic. With my first immigration wave, 7 dwarves arrived, 2 of them got incinerated because I didn't notice they'd turned up (I thought the wave had stopped). The third died when underground, the naked thing seemed to work fine for him, so don't know what happened, maybe he just spent a few too many seconds outside. All the animals died except one Gosling which promptly matured into a goose. She is gigantic and fat, which maybe contributed to her survival. Or maybe I just got her owner inside really quickly and she followed.
My second immigration squad, despite appearing somewhat further from a safe ramp (though in autumn rather than summer!), enjoyed a 4/4 survival rate, with no injuries. A boar also survived.
Anyway I consider Sol CONQUERED. With a safe method for bringing in immigrants, an underground lake, limonite, a massive spire of candy... not much can go wrong. I do want to see if a caravan turns up and if it survives to trade though.