That's because she's a ranger of Earth. She wouldn't leave it in peril.
It's also why Dekabright and Marigold didn't help or even get (Canon) ranger keys, since they're off planet.

No, no. I was referencing to the fact that her morpher was only supposed to work once. That it was for one emergency use. And she used it. So, I guess there was a loophole there, they could always recharge it...?

She doesn't have the same limit. That was SPD only.
She just doesn't use it often. It's more just a personal limitation than a physical one.

Deka Swan could only change once every four years (which is a weird number) and SPD Kat Ranger was supposed to be once, I believe, but that doesn't matter. All the teams that lost their powers came back, so why not her?

Well, in the Legendary War, Sentai version, Swan was there. Along with Doggy, MagiMother, Zuban, and a few other people. But they were fighting in a different location, somewhere in a forest on a cliff above the others. Not far off, but far enough to not actively be part of the battle used for our clips on SMF.

Even still, DekaSwan's 4 year time period was long since over, so she could morph once again.
So it's completely plausible for her to morph and fight.

Which number of Rangers are better and more enjoyable?
 

Is having 7 or 9 rangers for Super Sentai teams better than having 6 rangers?

I'll put it simple. Which mumber is better for a Super Sentai team? 7 or 9?

Swan didn't have a limit on her changes. She set the limit herself. She said she would only transform every for years, but she was ready transform again in the final episode- it's just that she was attacked before she could.
It is only natural that she would transform to help out in the legend war, because she was needed and wanted to help.

I have thought of a Made-Up Super Sentai Team of my own.

Desertranger

Rangers:
DesertRed
DesertGreen
DesertBlue
DesertYellow
DesertPink
DesertBlack (Hardcore Loner)
DesertWhite (Wise Voices of Reason)