Yellow Low Oil Light, Replacement Dipstick Funnel and topping up Oil

Apologies. Kinda 3 questions in one.

1. Yellow oil light came on. On checking oil level it looked correct but the dipstick and funnel disintegrated. Previous threads suggest this may be the sensor near the bonnet latch?

2. Bought replacement parts and fitted them as best I could. Problem is im not sure the funnel has been installed to its limit as now the dipstick reads about a half inch lower. Looks like I maybe low on oil. Which leads me to :

3. This car always had been serviced with semi synthetic oil for some reason, but, in not sure if my local indy mechanic followed this the last 2 services, he may have put in fully synthetic. Would there be any harm topping up semi synthetic on top of full synthetic? Could there be damage to the turbo down the line if I do this?