You mentioned a lot of stuff, so I'll just go through and do the stuff I have a definitive answer for.
when I step through the gate, half-life crashes when trying to load the pirate map.
It seems like either your changelevels are overlapping (this causes the game to load the previous map as soon as it finishes loading the new one, then loads the one it just left again, causing an infinite loop) or the landmarks don't match. check to make sure they're named the same and referenced as such by both changelevels.
Also how do you get more sprites to show up? I was trying to make stuff like torches with flames coming out, incense burners with smoke, and a fountain that spits water.
Also maybe a volcano that erupts every few minutes would be cool.
It sounds like you want particles! Spirit of Half-life, in addition to all sorts of useful features such as "movewith" (think Source's parenting system) and being able to use different models with NPCs, has a particle system you can use. It's basically a mapper's toolbox, and it requires no coding to get functionality out of (although the source code is available if you want to add even more stuff)
For example the pirate ship I made looks rather boxy and is "flooded" by the water it's trying to float on.
You'll have to use multiple water brushes, and move the one under the ship lower.
I was attempting to make a statue in the temple level, but something got goofy and it ended up blocking part of the map.
Just move it with the selection tool. Click and drag.
Oh and how do you turn gravity down?
trigger_gravity, I beleive.
[I was going to post a link to Spirit, but I can't seem to find the page I used to use. I recommend 1.5a4, because it's stable, official, (unlike 1.7, which changes a bunch of core gameplay elements) and doesn't have sparks over NPCs heads when they 'think' about stuff (1.8 does that)