>I wanted to get into concept art for entertainment design
Look at some concept art done for actual productions. Many of them are not finely finished art. They're not crap by any means, but they're done fast especially at the early stages. The important thing is to work out a design, fast iterations to refine it.
At later stages it gets more refined, and it can be rendered at finished quality for promotional materials. Say someone like Craig Mullens level.
http://www.goodbrush.com/concept/vbkex6jprsfk85yqyr042id07ji7br
But keep in mind something like that are rarely stage 1 concept art. pic related of a concept drawing that communicates an idea. The viewer didn't see the console in a lingering shot in the finished film, but some glorious bastard had to think of what and how it works, and drew it to communicate the concept.
>How necessary is practice and a preliminary portfolio before college?
Practice is forever until you're dead. Portfolio is needed when you're applying for a job, at the highest level your portfolio preceeds you. But you still need a quality body of work.
>How necessary is life drawing to the field itself?
If you're doing a lot of characters, it's pretty important. At the very least, you want to git gud at fast gesture drawings. If you get gud at gesture drawings, it can apply to almost anything else.
>How do I get gud at doing designs in the first place? Do I focus on line art and proportions first? etc.
Look at the elements of visual art and principles of visual art. Master all of them as best as you can.
>Is usage of 3D applications and photobashing acceptable early on, or should I practice challenging topics like perspective from the beginning?
Whatever it takes. Obviously don't do something like tracing off a 3D and claiming that was your own manual perspective drawing. But if you use a 3d app or photobashing to learn the fundamentals of perspective or whatever else, that's fine.