Assuming we're NOT talking about the Advance series ...
This is all off memory, so the specific numbers might be off, but this is what I remember:
1: 20% of the JP earned in a battle by a character 'spills over' onto the other members fighting in that battle, so with a full squad of 5 members, you're actually gaining 180% of the JP that appears on-screen (100% + [20% x 4 other members]). Spill-over JP is applied to the job of the character earning the JP.
2: No. Spill-over JP can take you all the way to Master level in any job. It's also possible to use crystallized enemies to master some jobs without ever touching it with ANY character, but this is generally considered a futile effort, as by the time you've killed enough enemies to do this, you could have mastered the job by playing it. It's worth noting that a crystallized enemy that has learnable skills will teach you ALL of the skills on the list, not just the one you select. The dialog that appears does not make that clear.
If your goal is simply to hit level 99:
The throw-stones trick is good for new characters, but extremely unadvised for levelling your top characters unless you go ALL THE WAY to 99 in a single battle, and then abuse saves to avoid random battles when possible.
All random battles scale with your highest level characters, they will rapidly begin slaughtering you due to having higher level equipment - and equipment is vastly more important than level in FFT. This can be offset by stealing the equipment off of them and using it yourself, but it's a time-consuming process to fully equip a squad that way. Levelling up by doing random battles and stealing all the equipment you can as you go will lead to better results than the throw-stone trick, but will also take longer.
If your goal is to master all the jobs:
Your first step on grinding JP should be equipping Gained JP Up (Squire support ability), and keeping it equipped at all times, as it doubles your JP earnings, including the JP spilled over onto other jobs.
If you're willing to abuse bugs in a game, there's also a couple of infinite-jp bugs in the original FFT. I can go into more detail if this interests you.
If your goal is to maximize your characters' potential:
Find a map with a degenerator trap and repeatedly step on it. These traps appear in fixed locations (there are five of them, I don't remember exactly where they all are). Degenerator traps cause your level to drop by 1. This is important, because when you gain a level, your character PERMANENTLY gains a slight increase in various attributes BASED ON THE JOB THEY ARE IN AT THE TIME. The actual formulas used at level up are extremely confusing, so I won't go into detail. When you step on a degenerator trap, these bonuses are not lost.
If you want a well-rounded character, do all your levelling as a MIME if possible, as Mimes have EXTREMELY good and rounded growth on level-up. If you want a high Magic Attack character, do all your levelling as Summoner or Black Mage. Basically, if you want to create a specialist character, do most of the levelling in that job.
If your goal is to make the game easy so you can see the story and then be done with the game:
Stop grinding. FFT is a trivially easy game, and can be beaten using some simple strategies available even in the early parts of chapter one thanks to broken game mechanics. I won't detail these strategies unless this is your goal, as I'm one of those people who can't resist using easy-win strategies once I know them, even if I want the game to be a challenge.
I've played FFT a bit too much, so feel free to ask any other questions you might have.