Check with the villagers to find anyone with decent engineering or scientific talent, and have them assist those efforts. (In the case of engineering, some able bodies may be able to help as well, even without significant engineering talent, at least for things like extinguishing buildings.) Instruct the experienced engineers and scientists to keep an eye out for genuine talent and useful ideas, but not to let ponies with more enthusiasm than sense slow them down.
For any remaining able-bodied ponies, (B), get them well-trained, with an emphasis on guerrilla and other unconventional tactics. This is not a time to train ponies in classic "line up and beat on each other in an orderly fashion" warfare. Again, watch for any natural talents and leaders here; look for anypony that others naturally look to without instruction or demand.
For anypony incapable of serving, either due to age or ability, have them start the communication campaign for any towns not already informed. Set up magical lines of communication (flame-mail or unicorn-based) with any major or minor population center.
On that same note, put the word out to Celestia's School for Gifted Unicorns, and any other similar institutions of higher learning (for all races), as well as the graduates of such institutions; offer opportunities both for any potential scientists and engineers, and for any potential mage warriors.
Keep the primary headquarters in Canterlot until there's a reason to do otherwise, and start fortifying it for long-term defense and siege resistance. We want to keep Canterlot as a visible symbol; abandoning it would send a poor message.
The messages to the Crystal Empire and the changelings should have informed them to start hunkering down as well, which will make them potentially viable secondary sites. (Both have the advantage of remote location.)
To clarify a previous point:>>762831>Diplomacy: (f) and (d), but if only one is possible, (d). See if we can't get some Crystal Heart involved here.
In particular, the Crystal Heart is a means of focusing love and positive emotions, which should be highly appealing to the changelings as a step toward peaceful and open coexistence. (Can't claim credit for that idea; I've seen it in a few stories.)
Also, while it wasn't a listed option, I'd propose setting up some stealth missions as soon as possible. Collaborate between the changelings and any unicorns capable of invisibility and long-range teleportation, with an emphasis on the latter at first to avoid risky infiltration. We need to start gathering information on language and culture; "extremely aggressive" or not, we need to hope that they have more reason behind them than just "exterminate". War is a last
resort, and understanding could help avert it. (I'm rather hoping that, despite the X-COM soundtrack link, the "enemy" is less implacable and inscrutable, and diplomacy is less hopeless than it is in that setting.)
(Setting question: are we roughly in the time period of the show, and in particular do we have access to some of the characters of the show? If so, we should start rallying them and consulting them.)