1. On wikipedia, it has a list of the names celebrated on each day. However, some names have more than one day each year (for instance, it says the name Michał is celebrated on March the 2nd and 14th, April the 10th, May the 4th, 8th, and 23rd, June the 20th, July the 5th, August the 25th, etc.). Would someone named Michał celebrate their name day on all those days, or do they pick a particular day and just do it once a year?
They only celebrate once per year. For multiple names, my family chooses the date/name that is closest to the birthday, but it might be different for others.
2. If someone was half Polish and lived in Poland all their lives, but had a foreign name (like Michel), would they celebrate their name day on the Polish equivalent day of that name (like on the name days for Michał)?
Yes.