Entangled: The character is ensnared. Being entangled impedes movement, but does not entirely prevent it unless the bonds are anchored to an immobile object or tethered by an opposing force. An entangled creature moves at half speed, cannot run or charge, and takes a –2 penalty on all attack rolls and a –4 penalty to Dexterity. An entangled character who attempts to cast a spell must make a concentration check (DC 15 + spell level) or lose the spell.
Grappled: A grappled creature is restrained by a creature, trap, or effect. Grappled creatures cannot move and take a –4 penalty to Dexterity. A grappled creature takes a –2 penalty on all attack rolls and combat maneuver checks, except those made to grapple or escape a grapple. In addition, grappled creatures can take no action that requires two hands to perform. A grappled character who attempts to cast a spell or use a spell-like ability must make a concentration check (DC 10 + grappler's CMB + spell level), or lose the spell. Grappled creatures cannot make attacks of opportunity.
Pinned: A pinned creature is tightly bound and can take few actions. A pinned creature cannot move and is denied its Dexterity bonus.. A pinned character also takes an additional –4 penalty to his Armor Class. A pinned creature is limited in the actions that it can take. A pinned creature can always attempt to free itself, usually through a combat maneuver check or Escape Artist check. A pinned creature can take verbal and mental actions, but cannot cast any spells that require a somatic or material component. A pinned character who attempts to cast a spell or use a spell-like ability must make a concentration check (DC 10 + grappler's CMB + spell level) or lose the spell. Pinned is a more severe version of grappled, and their effects do not stack.
Firstly, they don't seem to be typed "bonuses" at all, they are in fact penalties. Neither grappled nor pinned condition descriptions reference entangled at all. Without a statement such as the one pinned has about not stacking, RAW they would stack, but a DM might rule that grappled is a more severe version of entangled. IMHO.
they are penalties without a type, yes they do stack.
Multiple creatures can grapple opponents, that's actually how darkmantles usually fight. 12 darkmantles versus 4 pc's. 3 per pc.
Even if they were typed penalties, they would stack unless from the same source or otherwise specified. The general rule is that penalties stack.
The penalties stack - perhaps they shouldn't, but they do. There is some logic to it - if you're covered in gooey stuff (tanglefoot bag), or have vines squeezing around you (entangle), that's a bit different than a more conscious force attempting to restrict your actions.
Multiple creatures can grapple opponents
Not true really. Each extra creature just adds +2 to the first one's check. You can sort of 'chain' grapples (A grapples B, but is grappled by C) -- since the 'grapplee' does not have any required actions, they can perform or maintain a grapple on someone else.
[spoiler=PRD]Multiple Creatures: Multiple creatures can attempt to grapple one target. The creature that first initiates the grapple is the only one that makes a check, with a +2 bonus for each creature that assists in the grapple (using the Aid Another action). Multiple creatures can also assist another creature in breaking free from a grapple, with each creature that assists (using the Aid Another action) granting a +2 bonus on the grappled creature's combat maneuver check.
I am thinking of a level 4 Assimar Oracle of Battle with his favored class bonus going to the maneuver mastery revelation so he would be considered as a +6 BAB.
I, personally, would rule that that does not work. The effect of the Revelation is to consider use your Oracle Level for BAB, no matter what level of Oracle you are. Similar to casting transformation and arguing that a bonus to caster level increases your character level for the purposes of that spell. But that's neither here nor there.