.../... the hole into which they (appear) to need to be inserted, is only 2.5mm diameter.../..should I drill out the holes.../...
No, the large spring does not go there! That position is for the little, second spring. There were three major versions of the soft top frame in our cars (plus a fourth, very early version, see text in our Tech Manual under heading "Soft Top").
You have the second version. Pictures here (third version) show second spring attached between "A" and "B"; and you see by "A" is where it fits in the 2,5mm hole.
BTW, what happened to your original springs?
Garry's pictures of a 230SL (Reply #1) show the first version frame, which did not feature the second, smaller spring and which had snap locks on each side on the lower rear bow.
/Hans in Sweden