Heill Dan.

> There is one notable overlap - þ/ð.

Not so :-) Thorn and eth represent a single phoneme in Icelandic
and eth is often unvoiced. Using two letters in the spelling is
completely redundant. In the earliest manuscripts thorn was used
exclusively. Then eth had a century or so of usage before being
replaced by 'd'. The eth was resurrected in the 19th century.

> Although both are spelled 'th' in
> English, there is a phonemic difference (ie. breath/breathe).

Yes. Or 'thy' and 'thigh'.

> Now, I'm confused, though. How would a pair like tala/dala be distinguished,
> if not by voice?

By aspiration only.

> Also, I've never seen voiceless 'm', what words does it appear in?

In words like 'skemmtun' as opposed to words like 'skemmdur'.

> However, that being said, the loss of voice-distinction helps explain the
> development of the Younger Futhark...

I doubt that the voice-distinction was gone by then. It seems to be a later
Icelandic/Faroese development.

Kveðja,
Haukur