All church records were kept and (barring the gaps due to lazy vicars etc) are on microfilm so you have lost nothing see http://www.ee.surrey.ac.uk/Contrib/manx/famhist/genealgy/church.htm
The 1851 census would be based on parish boundaries which had not changed since 1154 it would seem so if their farm was in German then they should be found in Kk German register.
There are always exceptions - eg farms on periphery might sometimes use a nearer church in a different parish (there are cases where weather, floods etc made getting to a baptism impossible) but by law they should have used their parish church.

The parish reg (not the 1910 transcript which forms the basis of the IGI) should say (of this parish etc) as by church law they were required to - see my pages where I hope I have tried to make things reasonably clear