I've just used the urethane glue (Probond, i think) on my yak. It requires moisture to cure (ie: hydroscopic). I thought that was useful for 'steamed' pieces as it wasn't necessary to get them dry to bond. The glue is bubbly and expands, but beneath it's skin it appears more like urethane foam (ie: weak). Is this the same glue you are using? And why the high clamping pressure?
Also, I wonder about your water-based urethane. If the can says Varathane Diamond Finish, that would be an acrylic, slightly milky wet, drys clear, very abrasive-resistant. Varathane just ain't urethane anymore. Sort of like water-based contact-cement: greenish latex base. Flecto: Reid Paints International of England. Or is it something else and how is it standing up? taz