Hogs do sometimes decide to take up residence in feeding boxes. Sometimes for a day or two, and others to make a proper nest. I had one here who made the most intricate nest in one of my feeding boxes one winter. It filled the whole box with carefully woven grasses and leaves, with a small chamber in the middle for curling up in (which I discovered after it was vacated by the hog). In the end it decided not to hibernate, but used it for naps in between snacks. What was even more amazing was that it was a hoglet who had obviously never done it before, but just knew exactly what to do.
If the hog is only using it for a short term day nest, taking in just a few bits of vegetation is quite normal. If it was going to use it as a birthing nest I would expect it to take in more. But whichever it is, it’s best not to disturb the hog. With females, they may desert or kill the young ones if they are very small. But if a female is planning on giving birth, she might also move on if disturbed and then have to find another nest, etc. at the last minute – not the best. So it’s best to be cautious.
The huffing and puffing and circling is ‘courtship’ behaviour, but it doesn’t always lead to hoglets. But a pretty good indication that you have male and female around – so hopefully some hoglets at some stage.
I suspect the digitalis leaves were chosen because they were nearby? Like you, I would trust in the wisdom of the hog not to poison itself, but don’t have any information about hogs and digitalis. You might like to put some other nesting material i.e. leaves, grasses, etc. nearby, in case this is a long term stay and the hog might like some more ‘bedding’.
And you might, as I did, need to get a new feeding box!