Holy crap. Welcome to the world of Metroidvanias! It's probably my favorite subgenre of 2D platformer (and probably favorite genre of all). Something about the myriad powers, the backtracking that opens up new areas of the map, the quest to get 100%...
Anyway, before I lose my train of thought, I figured I'd help out a bit. It's been a while since I played so I'll do what I can.
- How do I use items in my inventory? So far the only method I've found is to equip the item in a hand, exit the menu, use it, re-enter the menu and re-equip my weapons.
As far as I know, yes. It's kind of a pain in the ass (as future Metroidvania games have evolved).
- What do these items do? Is there a good place to figure out what items are worthwhile and to tell me in what situations I should be using them?
They do all kinds of stuff. Some heal, some are equippable, some give you access to other areas. You'll just have to experiment and find out!
Or, check out this: http://www.vgmuseum.com/mrp/sotnweapons.htm
- What weapons and armor are good? It doesn't seem like I can just go by the stats, since things like weapon speed and very ill defined special properties of this stuff plays a HUGE part. Right now I'm using the combat knife, the icebrand sword and diamond plate.
Pick what suits your playstyle. Do you like to hit hard? Or do quick, small attacks work better? And while defense and attack are great, sometimes the attack animations or stat boosts of other items are better. You'll just have to experiment.
- What spells should I be using? (if any)
Here are a list of spells you can use: http://www.finalfantasykingdom.net/sotnspells.php
Again -- experiment. It's easy to get MP back, so just figure out what works for you.
- What familar(s) should I be using? I use the healing faerie mostly now. I also have the demon at the moment.
The faerie is good for defense, and as it levels up, it'll find hidden areas of the map for you. But the best familiar you'll get is the Sword. You'll see.
- What should I be doing at the vendor(s)? I've only found one and I think I've only visited him once. I'm not even sure I could find him again... Is it worthwhile to be selling stuff? Is the stuff he has for sale worth buying? Any recommendations on what to pick up?
Typically, the vendors are kind of pointless, though you'll find some worthwhile special items there -- new spells, a castle map, that sort of thing. Buy all the special stuff when you get the money for it, and don't bother selling stuff -- you'll find plenty of money as you go.
- Is there a recommended FAQ that gives me a good, well organized dissemination of this information?
Those links I've posted have some good info. GameFAQs is also helpful.
- Is there any general advice you can give me?
Explore. Remember where certain item gates are -- places that are obviously blocked off -- and try to return when you can. Use teleportation to get around easier. Don't worry so much about grinding -- levels will come as you explore and proceed with the plot.
Worry first about beating the game. You won't get the true ending. Once you've "beaten" it, you can jump back in and try to explore that castle for 100% completion. Don't be afraid to look online for a guide to get the secret items you need to unlock the SECOND HALF of the game. Yeah -- beating it the first time at 100% isn't the end.
- I'd like to explore the GBA and DS Castlevania's after I "grow up" on this one, will this give me a the skillset I need for those? Or will I have another learning curve for those games?
Absolutely. The GBA/DS games are largely similar to this in concept, with a few different gameplay tweaks here and there to change things up. Honestly, this is probably the hardest one out of all of them (they have varying degrees of difficulty, but are generally easier than Symphony). If you can get 200% in this game, you'll be able to tackle the other ones no sweat.
Also, someone made this awesome tumblr: http://metroidvanias.tumblr.com/
You'll find Metroidvanias of all stripes -- free ones, paid ones, console ones, PC ones, combat heavy ones, exploration based ones... you've entered into a rabbit hole, and I doubt you'll want to crawl out.
- How did anyone get through this game before the days of FAQs?!?!?
Dude, I have no clue.
Anyway, hope that helps! Enjoy! Symphony of the Night has its quirks, but it's a real trip, and I wish I could go back and explore it for the first time again.