It's not known- in-canon, the Citadel sits in a giant cloud most of the time. Probably just a screw-up by Production I.G.
Why were some of the Collectors like fifteen feet tall?
Probably an artistic choice by Production I.G to make them more intimidating.
They're soulless bug monsters that are invading, kidnapping, and grinding up humans, Production I.G. You don't need to make them more so.
In addition, the Reapers modified the Collectors significantly from the original Prothean baseline. It's possible that like the Collector-Generaly-type, there might be other varieties- a captain variation can be seen in Mass Effect 3, for example. A larger variant of Collector might be used for hauling cargo.
Why was Treeya an Asari? I know that seems like an odd question, but she didn't do anything a human couldn't have done. One thing that the Asari are known for is that all of them are biotics to one degree or another... which would have been massively useful at various points. She could have been replaced with a human with minimal differences. It feels like she was made an asari just so this movie could say "Look, this is Mass Effect!"
Not all asari are good at biotics- it's mentioned in several places that asari with underdeveloped eezo nodes or limited training aren't as good as those who do have properly developed eezo nodes and training.
Her being Asari meant that she wasn't paralysed with Vega's squad, so she could interface with the Prothean beacon and setup the Sadistic Choice. If she were human she would have been put in a pod with the others, not get the data, and thus mean that Vega wouldn't have anything to angst over, he'd be a hero who saved as many colonists as possible.
Treeya says that Liara was her mentor. Liara, who in the first game says that in asari terms, her age of 106 made her considered as barely more than a child, given that asari live to be at least a thousand. While asari are later said to reach physical maturity in their forties or so, and Treeya's age is never given, leaving the possibility for Treeya to be younger than Liara, the fact that we later see in Mass Effect 3 research notes of Liara's dated only a few years before the first game that indicate that she was probably not long out of university at the time of the first game - when would she have the time to serve as a mentor?