Bertwyna can be a modern form. Source: https://oswinkinsey.wordpress.com/ [See document titled "English Compound Names". 'Bertwyna' is at page 113])Based on endings of other Old English feminine names ending in -wynn (like -wyn, -wynne, -wen, -wena and -wenna), I imagine that Bertwyn, Bertwynne, Bertwen, Bertwena and Bertwenna could work as other forms. Unfortunately I can't find sources showing these though.
Based on endings of other Old English feminine names ending in -wynn (like -wyn, -wynne, -wen, -wena and -wenna), I imagine that Bertwyn, Bertwynne, Bertwen, Bertwena and Bertwenna could work as other forms. Unfortunately I can't find sources showing these though.