This course uses 'dore gurai'; I often hear and see 'dono gurai' which seems to be synomymous. Are both correct? Is there any difference? In usage (regional, age, sex etc) or 'nuance' maybe? Or are they truly correct and synonmymous?

They're synonymous.

They are both synonymous

Hnruttさん、 こんにちは。 Re:　どのぐらい　ｖｓ　どれぐらい Yes, the two have similar meanings so usually it can be used interchangeably. The slight difference in meaning is: どのぐらい can be used when you are not sure of the context(counter form). E.g. How long have you studied、How long something will take, etc Whereas どれぐらいC can be used when the speaker knows exactly what context/object they are talking about（and usually can see the objects to make comparisons). E.g. If you are at a store and asking to purchase a container of specific size from a row of displayed products, the salesperson may ask: どのぐらいの大きさが必要ですか？ What is the size [of the container] that you are after/in need? 大きさ＝おおきさ　（Size)　「noun] 必要＝ひつよう　（Need,essential, 「noun」