Regarding the lecturette you have to give at MB; could someone tell me if you're supposed to have this prepared before you arrive or are you given time whilst there to do it? Do you choose from pre-given topics, is it completely free choice?
If it'll help at all from the RCB time, the forms that were filled in and sent to Westbury prior to the Main Board included a part about hobbies & interests. At RCB itself we were given a choice of three presentations based on our hobbies and interests. I seem to recall we had an evening to prepare for the presentation itself.
If you want this badly enough I'd suggest there is no such thing as too much preparation and that if you have already sent on your Westbury paperwork, preparing a 5 min presentation on each of your hobbies will stand you in good stead.
N.B. Equally, if the format has changed hugely and this advice is superceded by someone far more current than I, then ignore everything I have said and good luck!
If you are unaccustomed to speaking out in this way, structure is your friend. Tell them what you're going to say, then say it, then tell them you've said it.
In the short prep time available, sort out the beginning and end and then prepare headings for the middle; for example, begin by saying you are going to look at the topic from three viewpoints, such as the impact on the individual, on the Army and on society. Or consider two opposing arguments about it; whatever is appropriate. If you are familiar with your material it will then present itself neatly and you will convey an ability to think in an organised manner.