Would it not have been better for european union members with stable currencies like germany to keep their own currencies? why or why not?

Better for whom? Germany's stable economy helps itself best if it keeps it's own currencies. This very question is what motivated the UK's Brexit.

However, if Germany and the UK, for example, deemed it beneficial to reactivate their membership in the EU on a temporary or, say, 'per their discretion' basis, for reason of providing and gaining greater purchasing power of goods from offshore regions, they should be given the latitude to pay into a large purchase under consideration by the remaining EU members.

This way, the stronger economies would not constantly be drained by the EU's poorer economies. This should not be the end of the story, however. The deutchmark and the pound would be better able to help EU from positions of strength if they decided to participate in a high volume, high value purchase. Adding the large German and British orders to the little orders from dozens of tiny countries would drive down offshore sellers' cost per unit, thus benefiting ALL the EU, Germany, and the UK.