Is it necessary to play all the games to understand the whole story?
To begin with, nope! Each game is pretty much separate from one another. The story tends to change, although certain themes exist. Sometimes the games do make references to past entries in the series, where you'll be like "Ahah!", but that's really it. What I will say is that some games are completely different. XI and XIV are pretty big in this, as these are MMORPGS, and not single-player games.
If the answer to the above question is no, please list all the FF games worth playing.
To an extent, I guess all of them would be viable. I think it depends how you find the series. For a lot of people, I know that VII was their first game, and they always speak highly of it. I myself started with VIII. I think Void is right in thinking that VI is a good starting point. It's a really nice game that gives you an idea of what Final Fantasy has to offer. I feel one that might be a right fit for a lot of people is Final Fantasy X (I genuinely thought VoidGear would have said this. Shows what I know, eh?) . It's a pretty great game that is currently available on PS4, and I feel it gives a really great example, just as VI does. VII is also accessible on the PS4, however, and definitely worth a shout considering how acclaimed it was.
Are there spinoffs of certain numbered games that are required to be played to understand the story of the games?
Certain games have a few sequels, if that helps. I believe IV had one. Final Fantasy VII is often referred to as the Compilation of Final Fantasy, and contains different video games and even a movie that furthers the game. X also has a direct sequel, referred to as X-2. As I said, Final Fantasy XI is an online game, alone with XIV. However, these games have expansions that further the game, so you could think of them as additional games, I suppose. Final Fantasy XIII also has two sequels, although I couldn't give you an idea of how those work, having never played it myself. Final Fantasy XV also has a movie to accompany the game, known as Kingslaive, and there is also a YouTube Series called Brotherhood, which lasts about 5 episodes.
Using the KH story as a standard, how convoluted are the stories in these games and will I have trouble following any of them?
I feel Kingdom Hearts is a lot more lighthearted than what you'll find in Final Fantasy games. The plot can be pretty convoluted at times. I think earlier games were a lot easier to understand, but further down the line, it does get a bit "what".
Again using KH as a standard, how difficult or complex is the gameplay, and how similar is to the KH gameplay?
I feel Final Fantasy XV is the game that would be closer to the gameplay style of the Kingdom Hearts series. XV is more of an action RPG, and can be pretty frantic. But the other games weren't really as hectic as that. For example, a lot of the games incorporate a turn-based system. However, later games also decided to operate an ATB system, where you wait for a little gauge before attacking. However, I feel only Final Fantasy XV falls under the kind of KH standards. Difficulty wise though, I feel they can be manageable. Some games are harder than others though. At the end of the day, a lot of it comes under strategy.
Finally, on average how long would it take to complete one of these games?
These games can take quite a lot to finish, from what I can gather. I myself have only ever finished Final Fantasy VIII, IX, X and XV. I feel XV was a pretty short game compared to the others, if you don't focus on sidequests as much.
Hope this provides you with better idea! Don't hesitate to ask if you need any more.