> 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:

Varð Guðrøðr
enn göfugláti
lómi beittr,
sás fyr löngu vas,
ok umbráð,
at ölum stilli,
höfuð heiptr½kt
at hilmi dró.
Ok launsigr
enn lómgeði
Ásu árr
af jöfri bar,
ok buðlungr
á beði fornum
Stíflusunds
of stunginn vas.

(although this is a normalized version and not a ms text)

Kveðja,
Haukur