Definitely possible to get a bib through a
travel agent - keep an eye out here and on runDisney Facebook groups. But also keep in mind that TAs now require your to book hotel and park tickets if you buy a bib through them for WDW races. That's new recently.
Are you planning on following Galloway's plan for the half marathon? I'd say just follow the plan as it's written - don't worry about getting ahead, especially since you are a newer runner. You don't want to increase your mileage too quickly, but you also want to make sure that you peak at the right time. Missing one week of training won't derail you entirely, especially since it's in December (when you'll still have a couple of months until the race).
I go to the parks afterward - like
@FFigawi said, it's good to move around after the half to stretch your legs and whatnot. But I'd guess the degree to which you can actually stay in the parks and walk around is different from person to person - someone who is well-trained and in good shape will likely last longer.
I'll try to remember to let you now if I see any bibs for sale!