Are there many species there that are specific to Iceland which would be harmed by lupines and pines taking over?
If it’s most an amalgamation of stuff that commonly found elsewhere I think it would be fine.
If pine seeds came to Iceland on the wind 100 years before humans got there it would have been considered native. Most the seeds of all the other stuff got there the same way I imagine, unless they’ve been isolated since the island split from a continent somewhere.