IMHO this is a good time for the Debian project to reconsider the decision to rely on systemd. If you also think so, consider letting them know.
In any case, IBM is very respectful of the health of the open source projects on which it depends. While the vestiges of Red Hat may become less and less attractive as a place for a maintainer to work over time, I do not expect IBM to exert any control over any of these projects in any meaningful way any more than it has over the areas that its current employees maintain. Look at SCSI in the kernel, or even RCU, both maintained by IBMers (two examples among many).
(I worked at IBM in its Linux Technology Center for over a decade)
Can they not be forked in the case of a sudden change of direction that Debian, or anyone else disagrees with?
What do you mean by "control"?
Forks historically only really work if the fork comes from within.