You can do DME changes on your own via the
website, so unless you have a real compulsion to speak with someone on the phone - you can take ownership back of that small piece of your trip. You'll get an email automatically about your DME reservation - it's really just a convenience that TAs offer to do on your behalf.
As far as Room Requests go, you can use a service like Touring Plans to fax in an additional request - but that's a total crapshoot in general.
I've had better luck (as a VERY part-part-part-time TA) calling in the TA line with my room requests for my reservations than doing them as a general guest. Every single time I've done a room request as a TA/via the TA line, I either receive the exact room I want or a comparable one. Not so much as a direct consumer. I've even had a client that has been so concerned, we've done a three-way call with the TA line and the booking agent was able to read off all their requests that I previously had noted on the reservation (low floor, bathtub, far from an elevator, near transportation - and a room number that I researched and suggested that met all the criteria).
If you're worried about making extra work for your TA, it's actually very easy on their part - one phone call (and there's never a wait on the existing reservations line) and they can handle both the room request and DME changes. If she's done a great job for you so far that you don't want to take away her commission, then let her continue to work FOR you to earn that commission which would otherwise just go back to Disney while you do the work.
Is she really hands-on with other things like your
MDE - ie. altering FastPasses and ADRs? THAT would be something I'd request to have back immediately or to be completely in control of. But the little things? Let her handle those.