shorts and a floppy hat for the South; thermals and something furry for the North
You'll be lucky to have a shorts and floppy hat day anywhere any month. #2 refers to "the north" but Myvatn and Dettifoss are well inland and fairly high up (over 300m), which is more to the point of why there was so much snow there that late. At a sheltered coastal town like Akureyri it won't be much cooler than Reykjavik in May. Although, that said, there has been known to be heavy snowfalls even at coastal locations in the NE in late May, but it doesn't happen most years. Gullfoss, although in the south, is also well inland and fairly high.
'end of the Winter season' (whatever that is).
April is generally taken as the last month of winter in a practical sense, though of course seasons don't change instantly nor on reliable dates.