What are the sources that would support the position that pants can be modest if: a) they are very loose almost resembling a skirt or b) by wearing a top that is long enough that it covers all the way down to the split in the legs.
Most authorities frown on women wearing pants, as listed in this answer.
However, some would support wearing pants, like those described in OP's generous description, if the alternative to wearing them would be skirts which are halachically unacceptable due to their immodest nature. (Source: R. Yosef, Yabia Omer vol. 6 YD §14. Note: in the new ed., the above added in a gloss that R. Henkin opined that pants which don't stick to the body [essentially outlining the figure] are permissible and can at times be more modest than some skirts worn to the knees.)
Tzinius is a subjective area of halacha, and what is considered to be modest depends on where you live, your community, and your own personal definition of modesty.
I suggest you talk with your local rabbi (or rebbetzin) and ask what is considered modest according to your community's standards.
This applies to men as much as it applies to women. A man might be permitted to wear shorts in one community, but if he were to move somewhere else this might be considered totally unacceptable.
The main purpose of tzinius is to be modest, and remember the definition of this word: humble. Its purpose is to not be flashy, and also to be respectfully dressed before G-d. Make sure also to keep in mind chukas hagoy, that can be a issue for some articles of clothing.
First, you wrongly suggest, that there's a clear definition of Tzniut in the Halachah - not true.
Tznius is all about one thing - not to attract (
heterosexual) attention. That's done by two things:
An objective covering of private parts: is about what actually can be exposed. Different communities have different views, but most Orthodox Rabbis agree that only the face, the neck, and the hands can be revealed. The actual boundaries of the exposure tend to expand in the last 100 years, but the idea holds still. You pass this test!
A "subjective" overall sex-appeal: that includes bright (esp. red-toned) colors, skinny and tight look, extremely fashionable, fancy accessories etc - anything that screams "look at me!". This is subjective because it depends on the surroundings - looks that are accepted in one community stand out in others.
For example, many Sefardish Rabbis criticize fancy Ashkenazi wigs as being the opposite of Tzanua, because they look so nice - they look much sexier than the natural hair it is supposed to hide!
You seem to pass this test also for our general society (but you wouldn't in Meah Shearim, for example!).
NB: THere are additional considerations that have no strict connection to Tzniut, such as Bechukoteihem (not to wear clothes that are distinctively goyish) or Daas Yehudis (not a clear definition of Jewish women tradition) and more.
In a lecture about a week and a half ago, titled "Kaddish in the Contemporary World", (available on YU Torah), R. Aaron Rakeffet quoted R. Joseph B. Soloveitchik about women wearing pants. The following is my transcription of the recording, beginning from about 1:13:43 into the lecture:
The Rav permitted pantsuits in Stern College. All right, it didn't catch on but the Rav said halachically he doesn't see what's wrong with it as long as it's made tzniusdikly, not tight. All right, this is the fashion women wear today, and fashion plays a big role, tznius plays a big role in what we consider fashionable. Meaning, you have to look where you're living, where you're living, the surrounding people... women today began in Beit Shemesh, it carried a little bit to Yerushalayim, that they cover their whole bodies like Arab women. The greatest rabbonim, including in the Eidah Chareidit, came out against it.