Mousers and other ways to enter a foe's square

Okay, let's lead with the relevant rules text:

Underfoot Assault (Ex): At first level, if a foe whose size is larger than the mouser's is adjacent to her and misses her with a melee attack, the mouser can as an immediate action spend panache point to move 5 feet into an area of the attacker's space. This movement does not count against the mouser's movement the next round, and it doesn't provoke attacks of opportunity. While the mouser is within a foe's space, she is considered to occupy her square within that foe's space.

While the mouser is within her foe's space, the foe takes a -4 penalty on all attack rolls and combat maneuver checks not made against the mouser, and all of the mouser's allies that are adjacent to both the foe and the mouser are considered to be flanking the foe. The mouser is considered to be flanking the foe whose space she is within if she is adjacent to an ally who is also adjacent to the foe. The mouser can move within her foe's space and leave the foe's space unhindered and without
provoking attacks of opportunity, but if the foe attempts to move to a position where the mouser is no longer in its space, the movement provokes an attack of opportunity from the mouser. This deed replaces opportune parry and riposte.

Now it's clear that this deed allows you to enter the space of any foe, as long as he's bigger than you. Normally you'd be limited to only entering the space of foes 3x bigger than you:

Moving through occupied squares
Opponent: You can't move through a square occupied by an opponent unless the opponent is helpless. You can move through a square occupied by a helpless opponent without penalty. Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.

Ending Your Movement: You can't end your movement in the same square as another creature unless it is helpless.

(...)

Square Occupied by Creature Three Sizes Larger or Smaller: Any creature can move through a square occupied by a creature three size categories larger than itself.

(...)

Special movement rules
Accidentally Ending Movement in an Illegal Space: Sometimes a character ends its movement while moving through a space where it's not allowed to stop. When that happens, put your miniature in the last legal position you occupied, or the closest legal position, if there's a legal position that's closer.

(...)

Big and little creatures in combat
Tiny, Diminutive, and Fine Creatures: Very small creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2-1/2 feet across, so four can fit into a single square. 25 Diminutive creatures or 100 Fine creatures can fit into a single square. Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can't reach into adjacent squares. They must enter an opponent's square to attack in melee. This provokes an attack of opportunity from the opponent. You can attack into your own square if you need to, so you can attack such creatures normally. Since they have no natural reach, they do not threaten the squares around them. You can move past them without provoking attacks of opportunity. They also can't flank an enemy.

Now, the CRB text has some contradictions: a tiny creature has to enter a bigger creature's square in order to make a melee attack, but the moment it ends its movement (right before attacking) it's moved back to the last legal square. I'm going to make two assumptions here:

1) Specific overrides general, and a tiny creature gets to make attacks before being ejected from a square it had to move into to attack.

2) The "three sizes bigger" rule should be interpreted to mean that not only can a tiny creature move into a large creature's space, it can even end its movement there and remain there. However, it's not allowed to remain in the space of a medium creature, because that's only two sized bigger.

Are you still with me so far? (If not, I'm interested in those issues too.)

Now my question is the following:

The mouser's abilities seem to hinge on an enemy attacking you, missing, and you then moving into his space. But if you're small enough, can you move into the space, and harrass? Or do you only get to do the underfoot assault stuff if you entered using that ability.

Let's say you're a Tiny Fox (Kitsune with feat), but the Large troll is refusing to spend his attacks on a not very threatening looking fox. Can you still move into his space (three sizes bigger) and harrass him?

Likewise, the Monkey Shine feat from Ultimate Combat seems to do nice stuff if you're within an enemy's space:

Monkey Shine (Combat)

You combine acrobatics and opportunity to devastating effect against your opponent.

Prerequisites: Wis 13, Improved Unarmed Strike, Monkey Moves, Monkey Style, Stunning Fist, Acrobatics 11 ranks, Climb 11 ranks.

Benefit: While using Monkey Style, if you successfully deliver a Stunning Fist attempt, in addition to the normal effect of Stunning Fist, you can spend a free action to enter a square adjacent to you that is within your opponent's space. This movement does not provoke attacks of opportunity. While you are in your opponent's space, you gain a +4 dodge bonus to AC and a +4 bonus on melee attack rolls against that opponent. If otherwise unhindered, the opponent can move away from you, but if he does, he provokes an attack of opportunity from you even if his choice of movement does not normally do so.

Normal: You cannot enter an opponent's space.

So do you only get the bonus to AC and to hit if you entered the opponent's space with a Stunning Fist? Or do you get them any time you're in the opponent's space, regardless of how you got there, with the feat providing a way to do so?

You can enter through either method, and you'd get the benefits of both the deed and Monkey Shine. The "while you are in your opponent's space" doesn't care how you got there. :)

As for being three sizes smaller: yes, for Tiny and smaller creatures, you can simply enter other people's spaces. You normally provoke, but you can. The reason I say this is that swarms are by definition squares full of Tiny or smaller creatures, and they "attack" by moving into their targets' squares.

If 75 Tiny creatures (one of the four squares of a 300-unit non-flying Tiny swarm) can enter and occupy someone's space, then it must (as part of this process) be the case that a single Tiny creature can do this as well.

It doesn't necessarily follow for creatures that actually get to "own" one or more 5' squares on their own, however.

The reason I'm not entirely certain of that is because the effects of being in an enemy's space, are in the middle of a paragraph describing how you might end up in that enemy's space. So I wonder if they also apply if you entered the enemy's space in a different way?