Two quickies here.
1. Does the restriction on needing 2 free hands apply to Humanoid creatures as in type or body shape?
2. Is it possible to deal a critical when making a grapple check to deal damage?
1. I assuming your wanting to know if a e.g. 4 armed creature could have 2 hands full and 2 hands emtpy to grapple with ?
2. can only crit if you are making an attack, causing grapple damage is an effect you can apply.
based off those two questions are you asking if a 4 armed monster can grapple an opponent and then make an attack roll to smash him in the face with a hammer while he's being held ?
or did I just extrapolate in totally the wrong direction ? ;)
Not so much 4 armed but take a Minotaur it's gut a humanoid shape nuts it's type is Monstrous Humanoid. If its based off shape then he needs his hands free or he takes a penalty if it's based in type he can (somehow illogically) grapple with out free hands.
As to the critical was just wondering before I sprang for a burst effect on my armor spikes since if you can't crit you can't get the burst.
1. I would say yes. Otherwise monsters that use their mouth for "grab" would be taking penalties.
2. From the way I read it you make the grapple check and then decide to deal damage, move the victim or one of the other options, so I would say that since it does not inherently deal damage that it does not crit. I may be wrong however.
1. i don't know, logically it should be type, but i think whatever you answer here will create problems... like aasimar being immune to the penalty when they should suffer from it or some non-human body shape being immune to it (because they aren't human shaped) even though they only have 2 grapple-usable limbs (and thus if one of those is missing, they should be in trouble). should probably have objective criteria re: body parts insteady of being dependent on such an abstract quality.
2. per RAW yes, crits are a sub-feature of attack rolls, of which CMB are a type of.
per SKR no, because the CMB text talks about how 20s are auto-hits, but otherwise doesn't reiterate all the critical hit text (albeit it doesn't say that doesn't apply, and since paizo acknowledges that CMB are attacks, and that attack relevant wording does apply to them in other cases, it seems clear that per RAW they still do Crit since that was never explicitly taken away from them... the 20 auto-hit is simply re-iterating and is true no matter your Crit Range, since it's actually distinct from the Crit rule, e.g. sometimes you can threaten a Crit but don't hit on a 19)
in any case, same goes for other CMBs that may do damage, e.g. Sunder (although objects are immune to Crit damage, Greater Sunder applies excess damage to the wielder/wearer, who is probably not immune to Crit damage)
1. for the minotaur, it seems reasonable that they could use their bite in some way for the grapple, but i would still require 1 hand free to not have a penalty.
(BTW, if you look at the RAW, i don't see anything stopping a human from grappling with BOTH hands full, they just take the penalty for not have 2 hands free... rather an argument for Grapple NOT using UAS, since your hands, and feat, and head, could all be full/restrained, and you can still Grapple per RAW)
2. i would go with applying the crit effect for sure, grappling for damage is kind of sub-par anyways, might as well let it work here, especially when you are doing weapon damage with it anyways...
As a tag along to this question, would a buckler impede a Grapple check with a 0, -1, or -4 penalty? Or perhaps something unforseen...