I would suggest reading some of the posts on here that others have written around getting into UX as they can be helpful to get more of an idea around this.
A. The work environment depends on what job you end up with. I've also heard that design agencies can be a very intense and stressful environment, they can also be more based around UI then UX. So first off, I think that you need to decide on the area that you are most interested in, then make goals and plans around that? As being in UI can be quite different to working in general UX. I can give you an example from being integrated in a UX team in a company that creates its own products. I'm lucky enough to be with a company who's culture I work well with, and a great team. At the moment we are working on a big project, so it is pretty full on, you have to be flexible and good at working to tight deadlines. It can be quite stressful but it is how you deal with that stress that is important. At the moment we are doing a lot of ideating, sketching, talking to SME's, interviewing users, creating personas, working to spec docs for features, coming up with workflows etc.
B. It doesn't sound like your background would be a disadvantage, as you've worked with both design and people, which are great building blocks. You just need the passion and the willingness to learn and grow your skills, there are some basic soft skills you need to have, but for the hard skills you can learn these.
C. There is a whole lot of discussion already on this forum around portfolios which I think you could benefit from. In brief, it is better to not just have a UI portfolio, you actually want to show your process, what went right, what went wrong and how you dealt with it etc. As well as your portfolio, make sure your CV is up to date, gives the information needed and isn't long winded. I'm currently helping out by looking at CV's at my workplace, and one of the first things that I've noticed (which surprised me) is that no-one mentions the user or even user testing!
D. Design agencies do give you an idea of working on a wide range of projects, but I don't think you have to go into them first. If the environment doesn't sound like one you would like, I wouldn't risk loosing your passion for this topic on a work environment that doesn't fit with you. Look for internships, offer to do some unpaid work, get up your experience in this way, or even look to apply for graduate positions in UX. With limited experience you generally need to work your way up the ladder.
Hope this helps.