I think that even if Xabi Alonso leaves Leverkusen, it is likely that he will go to Real Madrid, especially since Real Madrid have long been targeting him as a replacement for Ancelotti, whose contract expires next season. Moreover, Xabi Alonso is a former Real Madrid player so he already knows Real Madrid's culture very well. Even though Chelsea are also looking at him, I'm not sure if Xabi Alonso will accept their offer.
I don't think Pochettino will be fired this season because Chelsea's target is not to win the title this season, especially after Chelsea overhauled the team on a large scale and Chelsea has a lot of young players with very little experience, so Pochettino's job is to build this team to be more solid first. so that next season Chelsea can compete for the title or at least be able to appear again in European competitions.
This is what I believe as well. He has shown himself right at the most perfect moment, and he has the most perfect background as well. He played at Madrid, and he did awesome at a year, and overachieved with his team just as Ancelotti may leave. If Ancelotti leaves, and if Xabi is first or second at Bundesliga (and potentially cup winner?) then that means he will be offered the job for sure.
I can't even imagine how happy and terrified he will feel about that offer, I mean even when I get an offer from Real Madrid at football manager I feel super happy, to think that he will be doing fine there is beyond understanding, he will be super excited about the chance and also feel a lot of fear of being terrible as well.