Boric acid on the soil will not do a thing for spider mites, as they do not use the soil surface in any life stage.
In fact I would be very wary of using it near seedlings at all.
If you use alcohol in a spray, don't spray to the point of runoff.
Why don't you just use a product made for spider mites, like neem oil? It's much safer for the plants than alcohol spray, which can dissolve the leaf's protective waxy cuticle.