Often an intenstive of 100mW/cm2 is the most optimum for enabling complete curing of UV adhesivs.

UV rays having wavelength between 365nm and 420nm at 100mW/cm2 are best to achieve complete curing.
It is alright even if there is just one surface which is transparent.As long as UV rays can reach the bondline, curing will be initiated.
Yes.A range of Loxeal UV adhesives such as UV30-11,UV30-27 et al are approved for use with medical devices.
UV adhesives are highly reactive chemicals and curing can start sometimes even when they are exposed directly to sun rays,however lack of correct intensity and wavelength can cause incomplete curing and result can be less than proper bonding.Hence it is best to use the correct light source for best results.
Loxeal UV30-60 is a GEL type of UV adhesive for applications on vertical surfaces and or for applications requring non-migration of the product.