1.) Yes, but that's much harder to do than to also use Mastery due to diminishing returns on Dodge and Parry and due to how stat budget works.
2.) Mastery doesn't suffer from diminishing returns and it also gives decent amounts of block per point.
3.) Block cap is what you've already talked about (miss+dodge+parry+block == 102.4%). Once you reach block cap, it makes sense to start swapping out Mastery for pure avoidance (dodge/parry) since, as you pointed out, taking 0% damage is better than taking 60% damage.

1. Technically possible, but don't forget the base miss chance is 5%, so more like 48.7% parry + 48.7% dodge. This would be the technical ideal.
2. Mastery is more important because block gets you closer to the 102.4%. First off, parry haste doesn't exist anymore, so parry and dodge are for all intents and purposes the same (which means the one that has more points in it is farther into DR so it should be reforged away). Mastery is more important because, although at lower levels of dodge and parry where they will reduce incoming damage MORE than mastery, blocking happens more, making the incoming damage less "spiky". So you have a more consistent incoming damage, allowing healers to use more mana efficient heals to keep you alive.
3. "Block cap" is referring to the fact that, when you reach 102.4% total avoidance, getting more block is useless, as it is the first thing to drop off the table. At that point, you want ot reforge away mastery into dodge and parry, while maintaining 102.4%.
I hope this clears it up a bit

Okay just wondering. Everyone says intellect is the most important stat, understood!
Then haste is also super important as we have to use tons of holy lights.

But should u reforge all your mastery/crit into Haste? Just some of your crit/mastery. Or is it best to just keep it as it is, or will it depend on weather u are running 5 man hcs or raiding 10/25man?

You should pretty much always reforge Crit or Mastery into something better like Spirit or Haste. Personally, for heroics I would probably recommend Spirit, but both are good. Spirit is almost 3 times better then crit for both output and longevity, and almost twice as good as mastery. Haste is superior to spirit for output, but with a non-trivial cost to mana.

Getting back to delukz' question

1) If you were to actually get gear to a point where you would have 102.4 pure avoidance, then you'd be a tank your healers love and your dps hates.
It's important that tanks are getting actual hits because a significant portion of their threat comes from this. The AP increase through Vengeance and abilities proccing on receiving hits are necessary for threat management. (warriors and druids get extra rage).
So even if you could get to a state of pure avoidance it's probably one you would want to avoid (pun!) and instead find a suitable balance between avoidance and mitigation.
In reality, diminishing returns are going to make being entirely avoidable impossible.

2) sixcore seemed to have missed the point on this one...
Mastery gives you a flat chance to block. X mastery is Y chance to block.
Both parry and dodge however are on a diminishing curve. So your first X parry rating will give you Y chance to parry. If you already have a chunk of parry, then adding X parry rating won't give you Y chance to parry, it'll give less than Y chance to parry.
As you get more parry%, you need increasingly larger amounts of parry rating to add another percent of parry. The same is true for dodge.
That also explains the motivation for the reforge. If your dodge% is higher than your parry% and you're not block capped, then reforging X dodge rating into mastery will (due to the Diminishing returns) will be the better choice. The amount of dodge rating you're removing will reduce your avoidance (parry+dodge) by less than it would if you took away the same amount of rating in parry.

3) There is no hard cap on block. It's similar to Hit or Expertise. It is 'capped' because at some point getting more doesn't have any effect at all.

Being over the cap in anything can sometimes be a good thing if a certain mob mechanic reduces your abilities.
A mob doing a stat reduction could change things, as you loose Str (parry) and agi (dodge).
There have also been mobs with a higher-than-normal chance to hit.
I'm not sure if either of the above occurs in Cata instances/raids.
In any case it has in the past sometimes been beneficial to be slightly over cap, I'm unsure if such cases exist in Cataclysm content.

Right now, it will put you at 41% just like the old 1% BV metas. This appears to be a bug.

How does effective HP calculates now? I keep seeing higher ilvl paladins with lesser hp then mine, but with a bigger block %. I gemmed stamina till this point, but I'm not sure weather change them to masterity or sta/masterity gems. I'll defintely change some at least for socket bonuses what gives mastery, but still would be nice to know what is superior due to EHP values.

Regarding professions as a holy paladin, is it worth it to level up Engineering for the int springs/rocket boots (now that they can malfunction) or would it be more beneficial to pick up something else?

Regarding Engineering, I've done some reading in the other healing classes topics and it seems the best way to benefit from the springs would be to pop our mana gaining CDs at the same time (ie. Divine Plea) to get the most out of it, but does that mean I have to use Divine Plea on cooldown for maximum efficiency?

Are there better choices?

In regards to the Synapse Springs, some basic math yields 96 passive Intellect up time (480*12/60). Though if used in conjunction with cooldowns that are % based on mana pool it's going to have a greater effect. Seeing as the only other glove enchants are 50 haste / 65 mastery, as long as you're using your glove tinker efficiently it seems like a solid choice.

As for the Nitro Boost, they have the ability to kill you now. They can either launch you into the air or deal 120% of your hp in fire damage (though I can't find the spell effect for either on wowhead). Seems like a risk you have to decide to take. On a personal note, I can't think of a time in WoW raiding where you needed the speed burst if you were paying attention. That said, it can be a nice "ohgodohgod" button if you're willing to take a chance.

Perhaps this will be added to the Compendium at some point, but how much better is Int than Spirit for a holy paladin? I'm trying to decide between a DPS trinket with 216 int and a healing trinket with 216 spirit (Rainsong versus Harmlight).

I understand the math behind reforging crit/mastery to spirit/haste, but the tradeoff between Int and Spirit seems less clear to me. It sure looks like Int is just almost always better than Spirit if you have that choice. It seems like you can find enough spirit by reforging. Is this accurate?

Bonus follow-up: We should be gemming for Int exclusively, right? Ignoring socket bonuses except for meta, just like before?

The glove tinkers allow you to still have an enchant on your gloves. Divine Plea + Glove use is nice, but it seems like other professions are better since you don't have to worry about timing.