I'm not a lawyer, so I'm only giving you my guesses and 'safe bets'.
A) What about Phoenix Tears, Phoenix Feathers? Both have been used before, people will instantly know what they are for. Heck, deep fry the bird itself!
Seriously, what about Phoenix Hearts? Eating the heart of a dragon is supposed to give you wisdom. Maybe eating the heart of a phoenix will revive you from the ashes, instead of the phoenix being revived.
The key here, is you're too frequently stopping at 'copy' the well-recognized Final Fantasy things, and not moving beyond that to 'transforming' and 'combining'. I'm sure you're creative enough in other areas, but in the 'recognizables' of Final Fantasy, you're not wanting to move beyond copying, but that is precisely where the legal danger arises.
B) Common ailments I don't think are a problem. They are generic enough that hundreds of games use them. Whereas Pheonix Downs occur very rarely in RPGs, and are mostly only famous from the exact series you are getting inspiration from, ailments are used everywhere.
But don't stop at just the Final Fantasy ones - add your own as well. And mix up the terminology. Some games use 'Venom', 'Plague', 'Disease', as alternatives to 'Poison'. But really, 'poison' isn't going to get you into trouble. But be creative here.
C) "Potions" are fine. "Hi Potions" are an exact spelling of what Final Fantasy used (especially since you have zero need to abbreviate 'High' as 'Hi'). On it's own, any minor little thing like that won't cause you a problem. Taking together, dozens and dozens of minor similarities can work against you.
Elixirs, Potions, Draughts, Flasks, Drinks, Brews, etc... Red, Green, White, Blue, Rancid, Spoilt, Strong, Lively, Weak, Weary, Odd, Unusual, Transparent, Opaque...
It is zero difficulty at all to come up with new names. Many games use herbs. You could also consider powders.
D) Do you know how huge mythology is? Even if you avoided the Final Fantasy summons, you'd still have hundreds more characters than you'd know what to do with. But if you "just so happen" to, out of hundreds of mythologies from dozens of cultures, choose the exact same characters from the game you are inspired by, that hints of more than just inspiration.
If you were making an entirely different game, there wouldn't be any risk in using the same mythological characters that Final Fantasy happens to use. But you're making a game inspired by a Final Fantasy game, so you want to minimize similarities from that specific game series, and instead aim for generalities of the genre as a whole.
If you are lacking ideas and are only familiar with the Final Fantasy mythological characters, then go to your local library, and ask them for "Andrew Lang's Colored Fairy Books". This is a series of books by someone (Andrew Lang) who went and studied and translated hundreds and hundreds of different mythological stories from dozens of different cultures. Each book ("Green Fairy Book", "Blue Fairy Book", "Red Fairy Book", etc...) has fairy tales researched and translated from a different culture (meaning some of the books will have different cultures' variations of the same stories).
Don't be turned off by the term "fairy tales", these aren't just for children. Infact, when he originally translated them, he was verbally attacked for their utter "unreality, brutality, and escapism" that is "harmful for young readers". These aren't Disney's Peter Pan fairies, these are European Fay, which are closer to JRR Tolkien's Elves - but malicious. These aren't Disney's Aladdin Genies, these are the genies that flay the skin off people they are angry at. These aren't Disney's fairy tales. These are what Disney was inspired by, before Disney "cleaned them up" for a young audience.
Chances are, they'll already have several in that library. If they don't have it, ask them to get another library to send them the books. They'll charge you the shipping cost, but nothing else. Your tax money pays for the library, use it.
Alternatively, since the books were written a hundred years ago, their copyrights have expired and they are now in the public domain. You can download all of them, legally, for free, in PDF or other forms. If you got a Kindle, send the PDFs to your Kindle, and get reading. Or download from your Kindle directly, so they are free through Amazon's store also. And since they are public domain, if you want physical copies, you can probably get them for fairly cheap - especially if used.
If you're more interested in Middle Eastern mythology, try 1001 Arabian Nights. Also in the public domain. Also free electronically.
Reference the ideas, not the implementations. Make some great, something yours - not something living in the shadow of other people's greats.
I don't doubt your creativity, nor your capability. I don't think you are lazy either. The only question is, will you let your desire to play off of Final Fantasy's greatness hold your own game back from becoming something great in itself?