fsck speed mainly depends on the number of files and how they are spread in the respective directory. That said, 6 month for a fsck is absolutely absurd: it should had completed in some hours at most, especially if using xfs which has the speedy xfs_repair utility. Here you can find some fsck run at a scale – all completed under one hour (3600s). So, it is not possible that your fsck is still running.
Anyway, an unexpected power loss will not cause a full-blow fsck, rather only a very fast (some seconds) journal replay. However, if some key files was damaged, the OS can be unbootable.
But they probably just lied to you. You should stop paying immediately, ask for an explanation and apply for a total refund.