3oz/85g is practically nothing. Less than half an apple? Will you use the sleeping pad again? If so, I would go for the option you will get the most use out of. I get cold at night so would go for the insulation. Does it make the mat much bulkier if you add the insulation?
Google will give you average temperature charts. I assume you are walking the CF from SJPDP? If so, I think it’s probably worth thinking about how many kinds of environments you will be walking through. Mountains are mountains so expect anything. Galicia can be very damp and even if the air isn’t cold the dampness makes it feel colder than it is. The Meseta can be baking during the day and cold at night or it can stay warm.
For me, Early June, cheap summer sleeping bag, bivvy bag, wearing all my clothes, ancient thermarest, at Navarrete I was **COLD**. Outside Sahagun, same set up, I was warm.
I dunno about it being hard to pitch a tent, I camped in France and Spain with a bivvy bag, I wouldn’t have found it difficult to pitch a small tent discretely (except in the cities) but you have to walk in the evening, pitch as the light is going and get up when the sun rises. I wouldn’t bed down at 6pm near a village and expect no one to bother you! (The Spanish live a wonderful life and seem to stay up late and go for evening strolls). If you want to camp at a campsite your options will be much more limited. I believe wild camping in Spain is not legal but I don’t think anyone patrols the camino looking for illegal campers and most people find a quiet spot a little bit off route.(You can tell by the patches of flat grass)
Having said that, I did meet a guy from Belgium who had been camping/pitching a tarp in village squares for a long time, he said it was mostly ok but had some problems with kids throwing stones. (As a solo female I would not have drempt of doing this - out of sight out of mind!)
For info - People walk the camino at weird hours, I got woken up at 4am one morning by walkers talking very loudly. I had no idea why they were walking at 4am in the pitch black when the daytime temps were not going to be particularly high (they would probably have had no idea why anyone would camp either!) Given where I was they must have left at 3am....
FWIW - I usually get eaten alive by mosquitoes but didn’t find them a problem during my May/June and August. Flies and ants bothered me more. Unless you are very lucky you will get rain. Parts of the camino are wooded but a lot is fields grassland if you wanted to hook a tarp up to a tree.