Yes, both packages will be available. Mission Center is already published as a snap and is actively maintained, so users will be able to install it no matter what the default is.
At this stage, the schedule is quite tight, and the decision is unlikely to change even if all the accessibility issues are fixed and verified by the end of the year.
That being said, the accessibility support in Mission Center is still extremely important, so please do not hesitate to contribute to those fixes, they’ll benefit users regardless of the release of Ubuntu.
Thanks again for your help!