I'll try to answer the questions to the best of my ability, but I have a feeling it's all going to be answered by the time I type this all out. *weak laugh*
Stoneskin is modified by the Enhancement Magic Potency stat. Currently, the only way to increase this is through the increase of VIT.
This is referenced in this thread here:
http://forum.square-enix.com/ffxiv/threads/32606
(do a search for ≪Parameters and Their Effects≫)
Repose 'accuracy' is modified by the Magic Accuracy stat. This can be increased through the use of equipments, Materia, abilities such as Dark Seal and also through the increase of the MND stat, as referenced by the earlier link under Parameters and Their Effects.
A common misconception people have is the difference between the Magic Accuracy stat and the Enfeebling Magic Potency stat. Magic Accuracy affects the ability to 'land' spells, where Enfeebling Magic Potency stat affects the chance for additional effects to activate after an ability (such as the chance to inflict a Bleed effect after Aero or a Heavy upon a target after Stonera). This is referenced in this thread here:
http://forum.square-enix.com/ffxiv/t...%9B#post499398
Repose 'potency', on the other hand, I'm not sure with. I'm not even sure if it even exists because it may be (and most likely be) treated on the same scope as Sleep/Repose in FFXI in which its accuracy was the only measure of potency as its full duration was always static.
This is actually a difficult question to answer as it really depends on your play-style.
If you want to play it safe, stacking Enmity- is quite important. Because enmity in this game is calculated through raw damage and healing numbers (example: Say if you heal 1000 HP on another person without any Enmity-, the mob will have 1000 enmity on you, which can only be neutralized through another person doing 1000 damage, 1000 healing, or abilities that generate equivalent amounts of enmity), keeping the mob's enmity as low as possible so the mob does not come after you is a key thing to remember for any mage.
Healing Magic Potency, increased through equipments/Materia with Water Magic Potency (unconfirmed), Healing Magic Potency, or through MND which affects Healing Magic Potency is also important as it saves you MP in the long run as well as healing a person's HP as high as possible with 1 spell in a pinch. However, do keep in mind that because of how enmity is calculated, healing more isn't -always- a good thing because it means the mob will come after you faster if you neglect to control your enmity.
Finally, while I'm sure many will try to argue my point, while a maximum MP increase can be beneficial as you can cast more spells without rest from full, I would actually recommend maximum HP as the stat to increase compared to maximum MP if given a choice between the two. There are two reasons behind this. One is attributed to simple survivability. The more HP you have, the less chance you have dying in case something goes wrong. And two is because of how the CNJ's Blissful Mind ability works. When a CNJ is engaged in a long fight, MP management plays a much important role rather than maximum MP. Because the amount of MP you heal through Blissful Mind is dependent on a CNJ's current HP, this means the more HP you have, the more MP you recover, and in a longer fight, this will ultimately win out.
Other than this, there are other stats to keep in mind such as INT and Magic Potency (for raw spell damage), VIT (for Protect/Stoneskin), PIE (for Enfeebling Magic Potency), MND and Magic Accuracy (for accuracy in landing offensive spells or Repose), so there are a lot of things to consider. Ultimately, it is up to you to find out what's right what works to best accommodate how you play CNJ.
Yes, they can. However, unlike offensive spells/abilities, the log does not display critical hit cures. You can only spot them through random spikes in healing.
In terms of HP/MP ratio, Cure is -generally- more effective. I will only say generally because it might be possible with a high enough cure potency setup that the situation will reverse. Don't hold my word on this, however.
Sacred Prism + Cura will restore more HP than Curaga, but there's a huge drawback in the fact that it generates huge amounts of enmity. Because Cura will restore HP even when the character you heal has full HP, this generates enmity. To illustrate my point, let's say you heal 8 characters for 1000 HP each, despite the fact that 6 of them have full HP. This generates a total of 8000 points of raw enmity. Then let's move onto Curaga, where it's nature is that it will heal 0 HP if the targeted character already has full HP. Curaga will then heal the two characters that are damaged for a combined total of 2000 HP but heals the other 6 whom have full HP 0 points, thus generating only 2000 points of raw enmity. This is why it's generally a better idea to use Curaga to heal the entire party rather than a Sacred Prism + Cura unless a.) the mob(s) you're fighting are on the verge of death or b.) your entire party is in the red and you need to heal them even if it means having the mob on top of you.
I'm not sure how to answer this question as it's not exactly clear what you're asking.
Hope this helps!
- Stanislaw