If I can make a few comments about your dieing to the orcs.
Orcs are dangerous, but yours sound... worse. Can I ask a few questions?
1. You make no mention of armor. If your dwarves were running around in their raggedy clothing, it's no wonder they got ripped to shreds. Did you just not mention it? By the first invasion you should at least have leather (heck, in a "survival" map I played - starting with an axe and two kittens - I had leather armor before the orcs showed up) and if you build fast you should be up to chain or even plate from whatever metals you've found.
2. Did you draft your miners? If you draft a miner and leave his weapon as "unarmed" he'll fight with his mining pick, and use his mining skill as his weapon skill. Picks may be puny weapons, but a legendary miner can easily cut a swath through the orcs if he's got some good armor, especially since miners tend to be strong, tough, and agile.
3. How skilled were your original marksdwarves? You mentioned you drafted your peasants as marksdwarves, but how skilled were your original 8? An unskilled marksdwarf with wooden bolts will be lucky to kill a kobold, let alone an orc, while a legendary could easily put a bone through an orcs eye from 30 paces.
4. Since you were building an above-ground fortress, why not just wall yourself in? It's how I usually survive the first wave of orcs, they go away after about a season.
As for your questions, magma's easy, deserts vary, and glaciers are tricky.
Exposed magma has a tendency to cause brushfires from the imps, but once you have some infrastructure it's easy. Just floor over it or, if you have a stream, pump water over the top of it to form an obsidian cap. Bonus points for carboniting an imp.
Deserts can be really hard or fairly easy. If they have saguaro or underground water, wood won't be a huge issue, otherwise you'll have to trade for it. If they have a stream, water will be a cinch, otherwise you should write off any wounded dwarf, though booze production will be easy enough. If they have magma, forge-craft will be easy, otherwise it will be nigh impossible (forget about steel, you'll be lucky if you can spare enough wood to make iron armor).
Glaciers are hard, your dwarves will freeze to death and if you have no magma getting water will be a problem. On the bright side, you can build a palace of ice, which is kinda fun. Ice is even considered fireproof (the irony) so you can build, say, an ice furnace.