The effect of number on mutation in Welsh (for trochee)

trochee posted about this page of number rules in various languages, and asked why Welsh is so complicated.  I'm not a native speaker, but I'll try to clear it up a little.

As you probably know, Welsh has an extensive system of initial consonant mutation.  If you take the phrase Ceffyl yn yfed cwrw "A horse drinking beer", you may of course change it by specifying the number of horses, but you need to mutate the noun in various ways:

• Un ceffyl yn yfed cwrw — one horse drinking beer
• Dau geffyl yn yfed cwrw — two horses drinking beer [soft]
• Tri cheffyl yn yfed cwrw — three horses drinking beer [aspirate]
• Pedwar ceffyl yn yfed cwrw — four horses drinking beer
• Pum ceffyl yn yfed cwrw — five horses drinking beer
• Chwe cheffyl yn yfed cwrw — six horses drinking beer [aspirate]

This isn't quite the same as having a number system that goes "singular, dual, plural, and eight-or-eleven", since they aren't really new words, but rather mutations caused by the ordinary cardinals.  In addition, I don't understand why the Unicode rules give a special case for wyth "eight" and unarddeg "eleven"; as far as I know, those cardinals don't do anything particularly unusual.  Could a native speaker enlighten me?

The reason I said they were missing a case is that Welsh uses the singular form of the noun with cardinals below 100, but the plural at or above 100 (because e.g. 100 o geffylau "100 horses" is pronounced as cant o geffylau "a hundred of horses").

There is also the difficulty that pum "five" and deg "ten" cause nasal mutation only in the words blynedd "year(s)" and blwydd "year(s) old", so if your program talks about these then these also need special cases.

Some decent references for further reading:

• The syntax of Welsh, Borsley et al., 2007; see §5.3.1.
• Numerals, nouns, and number in Welsh NPs, Mittendorf and Sadler, 2005.

(One of the other thorny i18n problems in Welsh, which has nothing to do with number, is that the response to yes/no questions involves returning the verb.  So in a dialogue saying "Cau'r ffenstr hon?" "Close this window?" the buttons should say Cau "close" and Dim cau "don't close", not Ie "yes" and Na "no", which work but sound stilted.  There is no good provision for doing this in any existing i18n tool I am aware of.)