I'm confused whether you're asking if those things are possible to code, or if they were already coded, thus "possible for a non-hacker". If it is the latter, then it just isn't possible for any of your questions.
1. If I want to tie a status boosting effect to the terrain when moving over it, can it be done?
Yes, it would be possible... I would guess by adding the extra PA before the Attack Up checks...
2. If I want to tie an ability, which might be reaction or movement or support, that randomly checks if my character is suffering a set of ailments to pick one and cure it, can it be done?
It's possible, but I personally lack the knowledge on how exactly statuses are cured/added... it's more than just the stuff you can find in Battle Stats
3. The DEFEND and CHANGE EQUIP commands are added to the main set of actions of a character when equipped as support. Is it possible making CHARGE, JUMP or THROW work the same way?
Given the nature, I would guess this wouldn't be too hard to code.
4. Is it possible adding a new character animation tied to a certain skill?
You'd have to add the animations in TYPE1/TYPE2.SEQ, and even then, we're already pretty much maxed on space for these files... the SEQ files are loaded right next to each other, there really is no room to add anything unless you're ready to sacrifice another animation. That's why we weren't able to add an animation for Link's whirlwind blade.
5. Regarding STEAL command, can you set equipment values to whether they are more or less hard to steal?
I don't think it would be too hard to code it to load the target's item for a given slot and lookup its steal rate in a table.
6. Regarding editing a spell effect, I read somewhere that you guys still didn't figure it out how to make the screen or character flash. When editing a new spell sprite/animation, can we add a white squire and cover the screen with it by tweaking transparent settings with time lapse to simulate a screen flashing? I know this is a basic conception, but as I don't know much of freedom one can do, doesn't hurt asking, I guess.
This question is very confusing, unless you yourself, were confused when writing it so you didn't write it coherently to start with... I'm just going to have to assume "No, not possible".
1. As Elric explained to me, when adding a monster family, you change a monster which has 3 variants through color pallet. What I want to know is when you add a monster family in the place of another character, for example, Zalmo's slot, if that monster will have pallet parameters too, or if Zalmo is coded as a non-pallet unit, thus not allowing a whole monster family being set, only a single monster?
...I think it will work? You could test that yourself. I know the whole egg reproduction thing won't work, at least.
2. Is it possible making monsters equip items? Something akin to how default Reis base-job defines her.
Possible? Yes. Done? Nope. I think there are 2 things preventing this: The monster gender, and the Job Range (Mime to Tiamat)... meaning this latter portion has already been hacked and only needs to be adapted.
3. I gather that monster skill occupies a Skill slot, am I wrong? Is it possible using that skill slot to be used into a job ability set and vice versa (migrate a job related skill to become ONLY a monster skill)?
That question doesn't make sense.
1. Can we add a new ailment/buff? I mean, adding an effect completely, and not just erasing one to add another in its stead.
Nope. Unless you're willing to spend the next 10 years on this.
2. Regarding FROG, when you use frog on your enemy, your enemy still has the default movement (which means that a frog red chocobo will still move about the whole scenery). I want to know if when frog is applied, can you set a default movement to, say, 3 and jump to 6 and raise its evading numbers? Mainly is that I want to convey by adding frog ailment: it won't go too far when moving, still will be able to jump high and be harder to spot and hit.
That sounds like a lot of trouble for what it's worth, tbh. Possible, but troublesome.
3. Regarding SLEEP, STOP, DON'T ACT, can you make STEAL command 100% successful on a target inflicted by those ailments? I just think so strange the sleeping guy dancing about to evade thievery, or you missing the action on a completely paralyzed foe. And this would feel about my 5th question on the Skill section above, because even if your foe cannot move, some items are harder to simply steal. Of course, we can imply all this at the impossibility of stealing sleeping and stopped units, I just wanted to know if it's possible making stopped and sleeping targets easier to steal from at % odds of success.
That still requires hacking the formula, but yes it's possible.