Has anyone had success confronting their advisor about this sort of thing without being too hostile?
Fundamentally, I wouldn't view it as a confrontation. If someone is doing something unethical or abusive, then confronting them could be appropriate, but it's not clear that that's the case here. Instead, I'd approach it from the perspective that you'd like to learn what your advisor expects, his/her reasoning and motivation, and how much flexibility there is. Right now, you are unsure whether the amount of work required would be a month or a semester. That's a very reasonable thing to seek clarification for, and there are plenty of other natural questions. For example, how should you deal with any interruption to your primary work? Is there flexibility if you're at a particularly crucial or delicate point in your current project? Will your advisor help make sure it doesn't derail your work or delay your graduation? You could also ask about the motivation. If doing this doesn't seem valuable to you, instead of starting with complaints about that you could ask about structuring things to ensure you got valuable skills or experience out of it.
If you take this perspective, I don't think it will come across as hostile. You're just asking reasonable questions about what's involved and how it can be handled smoothly and productively. I hope the answers will be reassuring.
Of course this may be fruitless: your advisor could be a control freak or jerk who won't respond well to friendly questions. However, if that's the case you've got worse problems than just this issue. I certainly wouldn't start with the assumption that a confrontation is the first step.
It's also worth carefully thinking through why this arrangement seems unfair. As a mathematician, I have no experience with this style of research, but it doesn't sound unreasonable for the sort of lab in which students are assigned projects. You would be broadening your research experience, and your contributions would be recognized with authorship (but not first authorship since the senior student would have contributed much more to this paper, the same way you'll be first author on the paper that comes out of your main project). This sounds potentially worthwhile, so you need to articulate more clearly what's wrong with the particular situation you face.
If this is far outside the norms of your field, then I hope you'll be able to convince your advisor to do things differently. Otherwise, I'd just accept that you and your advisor don't agree about everything and, within the overall constraint of how the lab is run, focus on fine-tuning things to fit you better.
In fact, I feel like by bringing it up I could only make things worse.
This is the part that bothers me the most. Maybe you are just worrying too much, but it suggests something's wrong. Perhaps with how you tend to bring things up, perhaps with how your advisor responds to reasonable concerns, perhaps with your overall relationship with your advisor. Whatever the cause is, this is something worth trying to understand and fix.
In response to the "update" on the original question": I don't know enough about your research area to say how unusual or inappropriate that is, but it does sound worrisome. Do you have a mentor other than your advisor to discuss this with? That could be helpful for determining whether it's a major career danger or just a frustrating but manageable situation.
