I have had a hedgehog here all winter, and at various times mice and a rat. The mice here never seem to multiply and the most I have ever seen together are 2 mice – only ever 1 rat. They aren’t here all the time, so either go somewhere else or, anyway, aren’t multiplying, so I don’t worry about them too much, as long as there is food left for the hog.
Oats and blueberries, not a good idea. I would think they would be just as attractive to mice but not particularly good for hogs. I understand your feelings – being a vegetarian, but we can’t push our own preferences onto wild animals – they will eat animal protein whether we like it or not and it is natural for them to do so. I’m not sure what’s in Spikes, I’ve never used it myself. I tend to leave a bit of cat food out during the winter as well, in the hope that mice/rats are less likely to eat it and there will be some food left for the hog. So far it has worked – and by the way, I don’t eat meat myself either.
The best thing we can do for the hogs is to improve/increase their habitat by linking gardens and making our gardens more wildlife friendly. I have seen the hog here foraging on the grass (on video) for most of the winter as well as topping up with a bit of supplementary food.
When it comes to the warmer weather and you still have a mouse problem you could have supervised feeding times. I used to do that and the hogs soon learned when was the best time to come. It was a good way of keeping an eye on them too – and leaves them the rest of the night to find wild food for themselves – which is far better for them than anything we feed them. I feed the hogs on a lit patio and watch from inside the house (darker). (they don’t seem to mind the light as long as it’s consistent – they don’t like lights that go on and off so much, i.e. security lights) Rats and cats are very easy to scare away, mice slightly less so, but the hogs tended to turn up earlier in the night and in a fairly steady stream, so that the mice didn’t get much of a look in.
Good luck. Hope the hog continues to do well.