Can the Billy bookcase be shortened?

why not carefully removing the base and having the base run through a table saw to shorten the whole thing by the 0.53 cm (give yourself some clearance beneath the light switch. OR cutting the light switch. Id cut the wooden base cause I lik to mss with wood and when done It look like you planned it.

Or...
You could saw off the bottom of the vertical side pieces and the back the required amount, then drop it down over the base and reattach it. But that would leave a short bottom shelf.
So... You could saw off the top of the side pieces and the back, then reattach the top shelf. That would leave a short top shelf. More architecturally aesthetical.