Flowerfoot Swordmaster's last ability affects only Mice you control at the time it resolves, including Flowerfoot Swordmaster itself (as long as it's still a Mouse at that time.) Mice you begin to control later in the turn or creatures that become Mice later in the turn won't be affected.

You can pay an offspring cost only once as you cast a spell with offspring. You can't try to pay it multiple times to get more token copies.

If the spell is countered, the offspring ability will not trigger, and no token will be created.

If the spell resolves but the creature with offspring leaves the battlefield before the offspring ability resolves, you'll still create a token copy of it.

The token copies exactly what was printed on the original creature and nothing else, except it's a 1/1 (unless that creature is copying something else; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its types, color, or so on.

In the rare case where the original creature is copying something else when the offspring ability resolves, the token enters as whatever that creature copied, except it's a 1/1.

Many creatures with offspring abilities have other abilities that refer to them as "this creature" rather than referring to them by name. This difference is for clarity purposes and does not change the function of any of these abilities.

Any "enters" abilities of the copied creature will trigger when the token enters. Any "as [this creature] enters" or "[this creature] enters with" abilities of the copied creature will also work.

The token created by the offspring ability isn't "cast", so abilities that trigger when a creature spell is cast won't trigger for the copy.

In the rare case where the creature doesn't have the offspring ability when it enters, the ability won't trigger even if you paid the offspring cost.

Valiant abilities will resolve before the spell or ability that caused them to trigger.

If a spell or ability you control has one or more of its targets changed to a creature you control with a valiant ability, that ability will trigger if that creature hasn't yet been the target of a spell or ability you control this turn.

If you create a copy of a spell on the stack and target a creature you control with a valiant ability, that ability will trigger as long as the creature hasn't yet been the target of a spell or ability you control this turn.