[cypher]

I would stick with tried & tested things that are known to be safe, I love my ponal glue too I have to say & though a bit of an investment well worth it, Plastikote has been used for so long I would stick with that (there are various types of modge podge & I'm not sure which you were referring to but I wouldn't use that) for just one shelf a small pot of the Plastikote water based enamel is way more than enough & not that expensive really.
Not sure about hardboard tbh, I've never looked into that.
I always use dowels for legs on shelves, houses etc, you can get different lengths so may not even have to cut to size.