We've always taken the train on either a 24hr ticket or the weekend group ticket from Chopin to the main train station and the shopping centre. Pretty sure the same train goes up to Modlin and then back again.
Yes they have Polish passports and have lived in Poland for a number of years (even before the invasion)
They rode on MRF licenses back then but both now have Polish licenses as I understand
It's usually around the same weekend each year.
This is there website so you could keep an eye on there (uses translate to amend from Czech)
http://www.zlataprilba.cz/
Not sure you can do this as the Principality has electronic tickets only for entry as far as I'm aware as entry to the rugby matches are only on phone apps