How did B'nei Yisroel go from the depths of the '49 gates of impurity' of Egypt to such a high place that 'even the maidservant had nevuah higher than Yechezkel'?
Every Jew has a neshama which is part of G-d, and is always yearning to do the will of HaShem. When we do something out of line, we are always able to do teshuva because we know that our actions come from a place of wanting to do the right thing, even though sometimes a 'foolish spirit' can lead us to do otherwise. There is a part of a Jew that is never touched, that can never descend into the full impurity of Egypt and is always accessible. This essential truth helps to understand how the Jewish people accessed such great heights, because they were truly connected all along.
This is a huge part of the understanding of the full redemption. We need to always know that we are worthy and the potential for growth and renewal and greatness is essential to our being. This is also the message of Tu B'Shevat! Even though we can't always see it, we know that the sap is rising, there is greatness blooming. It is important to live with this faith and the mindset of Geulah imminently coming.
And finally, Miriam represents all of the Jewish women's strong faith even through dark times. It is up to the women of our generation (and the men, but women have a special energy of faith) to bolster and inspire this faith and responsibility to all of B'nei Yisroel that we are worthy and loved.
What is the significance of Rosh Chodesh being the first mitzvah listed in the Torah?
We know that within time itself are certain 'energies', that even the Avot were able to celebrate the holidays because of a certain potential that every time has to express that specific holiday. However, the Jewish people actually have been given the ability to "create" and sanctify the new month. This has a powerful lesson in it - that HaShem gives the Jewish people a large role in the world.
The Jewish people are to HaShem like the sun is to the moon, so to speak. The sun is always in its perfection and completion. However, its light is only reflected into the world during the night according to the amount that the moon is reflecting. So, while still for a short while we remain in this galut, it is up the Jewish people to reveal the G-dly light inherent in all of existence. That is why it is understood amongst the holy seforim that the Jewish people are the embodiment of the Shechina, which allows the infinite light of HaShem to be expressed in this world.
As an ending point, we know that Rosh Chodesh is specifically a women's holiday, as the women remained faithful to HaShem during the golden calf incident. When our emunah is strong, that even when the moon is just a sliver we know that it will soon be full, we are able to reveal the G-dly light within the world.