"Prynhawn dydd Iau"
Yes, Iau is pronounced the same way for Liver, Jupiter and in "Dydd Iau". It is pronounced as "Ya-ee" as you suggested.
Well it varies around Wales, but the southern pronunciation is "ee". I am northwalian so I pronounce it in the same way the French pronounce a "u".
If you go to the Welsh Plus pronunciation videos that we recommend in the pinned discussion 'Pronunciation - Ynganu' you will find examples of how u is pronounced.
Look in the discussion for the specific link, or look on the web for 'welsh plus pronunciation basics youtube'.
In this example there are three vowel sounds being run together.
At the top of this page, click on 'topic Welsh', then on 'Popular' and then on the second 'sticky' discussion 'Pronunciation Ynganu'. In MrGWallCymraeg's first post there is a link to a very good set of short Youtube videos on pronunciation. Watch them all!