Didn't see anyone have trouble with these so I am a bit worried about my intelectual capacity here, but... What are the exact limitations to their abilities?

The Beastman text reads: "Deal 1 dmg and then draw and resolve a new Fate card (limit once per Beastman)"

The Great Wurm text reads: "Deal 2 dmg and then draw and resolve another Fate card (limit once per unit per battle)"

Say I have 3 Beastmen in a battle. I draw 3 Fate cards for them, one of which is an Orb/Special Ability. I deal 1 dmg to the enemy and draw another Fate card.

Question 1: Acording to Special ability text, I resolve the new Fate card before revealing my other Fate cards I have already drawn?

Question 2: If the new Fate card is an another  Orb/Special Ability, what do I do? They way it is worded I can interpret it in three ways:

a) I deal dmg but do not draw anoter Fate card because this beastman has already triggered his special ability (limit once per Beastman);

b) I do nothing as this beastman already has triggered his special ability;

c) I deal dmg and draw another Fate card, and I can trigger the same effect a number of times equal to the number of my Beastmen in this battle.

Question 3: What is the "unit" in Great Wurms text? Can I trigger this ability as many times as I have units in this battle? Or is my enemy forced to assign the 2 dmg to a different unit each time I trigger it, and I can trigger it only as many times as my opponent has units (this would make sense fluff wise as the ability is called Plague Breath)?

I am confused...

As I remember about beastmen, the right answers to your questions are:

1) Yes (you are resolving the damage from special ability first)

2) The option A is correct: you deal 1 damage plus 1 more, but don't draw any additional Fate card (as it's limited to 1).

3) According to this logic, bone dragons should deal 2 damage plus 2 more, and don't draw additional card the second time. Still it becomes too powerful (e.g. comparing to the Elven trent hex unit), probably whyrms should deal just 2 damage and draw an additional fate card (but do not deal the second 2 damage in case of a second orb?

Hmm, good point, I guess the "limit" bit in the text means: "limit one special ability per unit", so I would opt for no dmg from a second orb in both cases.

However I am still confused by the difference in wording... clearly they could have fitted the words "Great Wurm" instead of just "unit" so that it is consistent with a previous similar working ability...

In both the Great Wyrm and Beastman cases, what the ability is trying to say is that a second orb just deals damage, but does not trigger another card to be drawn.  The damage for the second orb would still apply.