... and there was an upbeat, if determined, mood
Solution 1:
The meaning of "if" in "if determined", is "even though".
Merriam-Webster's 4th definition of "if" is
4: even though : although perhaps an interesting if untenable argument
At the same time, they were upbeat - enthusiastic, full of hopeful energy - even though determined. It's not an aimless energy, but grounded on a determination for change. Conversely, it's not a grim, gloomy determination, but infused with enthusiasm and energy.