I tried this against 1.5, on FF3.6/mac.
1) Runtime error: 'node is undefined'. Removing the attribute value='null' and replacing with defaultValue= resolves the runtime error.
This didn't reproduce for me, using your test file against 1.5.
2) If first item is selected in drop-down, neither event handler is triggered.
You should only be using onChange, not the DOM event onchange. Anyway, on selecting the first item, I see a console.log() that there was an onChange event.
3) If a user clicks the drop down, doesn't make a selection, and then clicks elsewhere on the form, clicks submit, dijitform.validate() passes even though the user never made a selection to a required field.
Aappears to be fixed. I opened the drop down for the first select box, but then clicked elsewhere on the page to close the drop down w/out selecting anything. On pressing "submit" it displays an error message.