1) I think we ought to trust Eric on matters Welsh.
2) [DISCLAIMER - I am not a linguist]
I have no problem with digraphs treated as single letters, having a distinct pronunciation, having a separate place in the alphabet:
a, b, c, ch, d, dd, e, f, ff, g, ng, h, i, j, l, ll, m, n, o, p, ph, r, rh, s, t, th, u, w, y
and collated accordingly, so that, for example, fy comes before ffrwyth in the dictionary
being called "one letter".
In other languages they might use diacritics to indicate pronunciation - in Welsh they use digraphs.
The German 'ß', e.g. Straße is printed as 'ss' when 'ß' is not available - Strasse, but that's not the same as Welsh - 'll' is not used in place of a single Welsh character which modern typography cannot easily accommodate - it's a different letter of the Welsh alphabet in its own right.
[/DISCLAIMER]