My Latin is also rudimentary, but from what I can make out, after their military training Henry and George went "beyond the columns of Hercules" (that is, into the Mediterranean), from whence they returned; then "Belgium called" (that is, they entered the services of the Dutch army fighting for independence against the Spanish). "Hvisdoni" is capitalized here and certainly a proper noun, and is also the place where "fiery death seized" George [might this refer to a fever?] - no clearer where it is, though definitely connected with the Dutch cause.
The main thing that bothers me about the memorial is that it puts George's death on 3 June 1624 - which would be before he wrote his codicil in January 1625 (and also before the siege of Breda began in August 1624). Note that 3 June 1625 is the day after the surrender of Breda.