This should actually be chwe chwningen - see the notes for the 'numbers' section which explain that pump, chwech, cant become pum, chwe, can before nouns. It also highlights the error in some questions such as this one.
There is also an aspirate mutation after chwe and tri.
Can't you get rid of these sentences with chwech and pump instead of chwe and pum? I just got this one as an audio exercise, and it pains me to type out something I know is wrong.
No, in Welsh we use the singular nouns with numbers. Also the plural is "Cwningod".
It is definitely there as one of the preferred answers. You may need to make sure that you have a capital Ch in Chwech - Duo seems to insist on the same capitalisation as in the database sometimes.
If you are using the app, make sure that you have the latest version - they often issue updates to fix bugs.
Apparently 'cwning' was a borrowing from Middle English 'conynge' (whence En. 'coney'), which, like 'kanin' ultimately derives through L. 'cuniculus' from Gr. 'κόνικλος', which itself is probably of palaeohispanic origin, possibly cognate with Basque 'untxi'.