There's no strong balance reason that they shouldn't stack. Demonic forms, and especially Processes, are supposed to be extremely powerful, with proportionately great risk when you resort to them. Even ignoring witnesses and Cover compromise, a specialized demon is still limited by the number of form abilities he can have, and gives up the versatility of all the other potential powers for one niche expertise.
Of course, if he's a powerful grappling specialist, he may not want to avoid being caught in a grapple in the first place, since it's where he has a significant advantage. So whether he has +3 or +6 Defense against an enemy pulling him into a grapple may not matter if he willingly forgoes Defense, just so he can turn it around on the enemy with his Unyielding Vice and Extra Mechanical Limbs. ("You can choose not to apply your character’s Defense against incoming attacks.") The Escape maneuver might be similarly rarely used.
Regardless, you don't even need to worry about Adhesive, since it doesn't actually provide "+2 Equipment bonus to grappling" or anything. Its only relevance here is
grapple, this substance can serve as equipment when restraining a target, in which case the Durability of the bonds cannot
exceed the demon’s Primum.
The secreted adhesive has no impact on the dicepool for grappling. It just provides free equipment to use when performing the Restrain grappling maneuver (DtD, p322):
• Hold your opponent in place. Neither of you can apply
Defense against incoming attacks.
• Restrain your opponent with duct tape, zip ties, or a painful joint lock. Your opponent suffers the Immobilized Tilt.
You can only use this move if you’ve already succeeded in
a Hold move. If you use equipment to Restrain your opponent, you can leave the grapple.
It provides +3 when the glue is used as a "tool for a relevant task," but until you've already successfully restrained the enemy's limbs and started applying the adhesive to them, it's not actually relevant, so it's no help in the task of overpowering an opponent.