Sahil is given a commission of lieutenant. Because he never went through the academy, he should only be able to join Starfleet as a non-commissioned officer, similar to Chief O'Brien from Star Trek: Deep Space Nine (1993). Non-com officers have a different ranking system from commissioned officers, comprised of various levels of petty and warrant officers.
Michael's plan to escape from the Emerald Chain is to eject and detonate Discovery's warp core, then have Book attempt to use the spore drive to jump back to the nebula. Discovery has been shown to be able to make spore jumps even from within an enclosed space. If Book can use the spore drive, there's no point in first ejecting the warp core.