Removing roof aerial

MrAbc

Hi all,
Hit a snag with what looked fairly simple...

The car is a relative's Nissan Almera '00 (N15 version it seems) and the aerial is the telescopic type that (manually) retracts into the driver's A-pillar. There's a plastic aperture screwed into the roof[two (+) heads] through which the aerial passes. The original aerial got broken off a few months ago and they got the replacement one recently and asked me if I could fit it.

Basically, [see attached sample internet picture] the two Philips-head screws holding the plastic aperture onto the roof proved impossibly tight :-( They just turned a fraction (maybe a 1/16 of a turn) and won't budge any further. They only tighten again. I rounded one slightly - can Dremel it into a slot-head later if needed.
I'm wondering, with the aerial broken off, did some moisture get in and rust the free ends of the screws... or... might these actually be bolts with nuts that need to be loosened by accessing inside under the upholstery roof trim?!

Any advice on the typical (or specific!) answer would be handy. Save me the cost of getting a Haynes manual or sending it to some garage :-)
Seemed simple DIY and it does look as if a bit of patience with WD40 could shift them but I don't want to round the screws and make it messy.

Many thanks in advance.

MrAbc

Any ideas even?