> While I knew about the 'ø' I was assured the extra on the end was incorrect
> for a singular ON forename.
> How was I mislead ?
I don't know. Maybe you're affiliated with the SCA. There seem to
be a lot of people there giving out bad advice about final r's ;)
The name Guðrøðr is certainly attested with a final 'r'.
The oldest example is possibly this stanza from the Ynglingatal:
sás fyr löngu vas,
at ölum stilli,
at hilmi dró.
af jöfri bar,
á beði fornum
of stunginn vas.
(although this is a normalized version and not a ms text)