The ratio of pigment (the color itself, which is the expensive part) to oil is lower, so you get weaker hues compared to artist grade, which usually uses the lowest possible mixture of oil for a given pigment. On top of that you get filler, the combination can lead to poor working properties like streaking, gummyness, etc. This is less of a problem with cheap pigments like siennas and umbers.
I’ve “recycled” acrylic painted canvas, but oil would make a fresh coat of gesso stick poorly. Canvas isn’t all that expensive, though. If you want, you can gesso the back side and paint on that.
Acrylics and watercolor paints are cleaned with soap and water, and there’s nothing different about the glue in the ferrule. The best advice is to make sure you don’t get paint so far up the bristles as to touch the ferrule. I personally wreck my brushes when I paint, so damaging them while cleaning them is their least worry.
The Girffin line are alkyds, and they’re not water-soluble. You still need a thinner to clean them. They are artist grade, but not quite as nice as the Artists’ Oil Colours. My father liked them and used them with good results. Alkyds dry faster than oils, in days rather than weeks and months, but not so fast as acrylics (minutes to hours).