An alternate perspective:
Prof A: Would you like to give a lecture on Topic A this semester to my survey course?
Prof B: I would like you to be one of our presenters when Visiting Faculty X comes to speak.
Profs A, B, and C: I think that this paper topic has the makings of an article and I would like you to aim at a publishable paper as your final project for this course.
I would only consider one of these a CV builder. A number of your ideas about what these opportunities signify is a bit naive.
I guess the situations with which I'm currently struggling most are those when a professor doesn't ask if I can do something but tells me to do it or takes it for granted that I will do it. For instance, I got an email not long ago that said, "Our speakers for x will be..." and listed me as one of them, without confirming that I was available for the event (let alone available to prepare a presentation).
Yikes. While it's nice to be flattered, you are having responsibilities
delegated to you.
In addition to all of the terrific thoughts above, I might also suggest that you check in with your actual advisor--at least briefly--for some help in prioritizing these things.
It is in your interest to ask about the culture of your program from a more advanced student before you get too comfortable picking and choosing to suit your research agenda. If there are tacit expectations in your program they are not going to be known by the fora.
1) Short term ideas: For the last-minute "assignment" you 'feign' elaborate surprise (true enough) and confusion..."I'm very sorry, I didn't have a record of that request and I am already booked for another event." (even if it's just sleeping to be ready for the ones you DO have a record of). People will learn you plan your time well in advance and are already a sought-after speaker; they will, as said above, respect you more.
It is not in your interest to beg off of one engagement with another that doesn't exist, or isn't scheduled for the same hour.
For the unexpected assignment, likewise, "I'm sorry, I will be giving a departmental presentation just before that class; I expect to be on time to class but won't be able to prepare anything more extensive in the morning." (Subtext: You clearly don't expect me to drop everything tonight and do this, so you must be thinking I'll have time in the AM; but it's OK, you couldn't know I had this other thing going on...)
Suggesting that something other than your coursework is more important than your coursework in the first year of a PhD program is not a smooth move.
2) People sometimes "float" a request just to see if you are available, and will cope if you're not; they sometimes get confused as to whether they already asked you, or just meant to; if they're under a lot of pressure they may refer to notes they made earlier that make them think they already confirmed the presentation with you and you just have to do the reality-check for them in a kind, professional way.
If someone is delegating to you, find out if this is par for the course. I do not recommend reality checking your faculty. I would express some concern about your ability to meet deadlines. On a recent thread, someone had a very smooth way of dealing with an AWOL dissertation advisor in this way.
3) As stated above, you want to accept those things that further your own goals. It sounds as if perhaps you need to map out a research agenda (this is something people often do later, but your MA period is not too soon) so that your criterion for accepting requests will be guided by knowing what you want for yourself and how that presentation can fit into your own larger plan for your work.
Repeat: It is in your interest to ask about the culture of your program from a more advanced student before you get too comfortable picking and choosing to suit your research agenda. This is very much along the lines of not participating in the life of the department.
That gives you a better sense of how deep you'll swim, where the shoreline is, and what course you're plotting to arrive at your own research island without drowning in the process. It also will guide you in how to subtly shape topics you present on, so that they serve your needs as well.
This is called an "MA thesis."
It can be done in a straightforward way, sit down at the keyboard and bang out an outline of what you want to do in the next 10 years, say. Or it may be the focus of a two-day "research retreat," perhaps with someone you trust as a friend to your work, in which you take some time to really think through your goals and integrate them with your short-and longer-term planning.
Generally, this is not a task for the first year of a PhD program.
I try to do the latter at least once a year, to catch up on where I am and what I'm doing; others never need to, your choices will be formed by what makes sense to you.
This is often called "annual review," is conducted by faculty, and addresses your progress towards your degree.
