Yes, it sounds as if they may have hibernated, albeit a bit late. I still have my visitor – she is quite big now. But she is a bit of an exception, in that not only did she not hibernate last winter (she was a hoglet then) but she also spent months in Wildlife Hospital after a life threatening strimmer injury and added to that she has poor, if any, vision. So I’m quite pleased she hasn’t hibernated again, so that I can keep an eye on her. I don’t see her every night, although I do most nights. That could be partly due to her managing to evade the cameras sometimes, though! Luckily, now, she is coming early enough for me to see her in real time, which is helpful. One particular cat, however, is not helpful, which might be why the hog is now visiting earlier, so that she gets to the food first!
Re. cleaning out boxes this is the recommended guidance https://www.hedgehogstreet.org/cleaning-out-boxes/
which says April and October. However, if you are absolutely certain they are empty, i.e. that it isn’t just in the entrances where there are spiders webs, you could give them a clean out now. But if you aren’t sure, be very careful, for fear of disturbing a hibernating hog. They can be completely covered in vegetation.
It won’t be long, now until the hogs start coming out of hibernation. The males tend to return first, often March, although it’s possible it may be earlier there. It’s also possible that as they went into hibernation later they might come out later as well. There is a certain amount of variability and individuality and also uncertainty in the knowledge about hibernation.