Hi FadedKnight,
You sound as confused as I did when I started all this
I'll try and answer some if not all of your questions and I'm sure SKS (just found out that means Some Kind Soul) will add to it.
A good place for you to start is to study and surf the GENUKI Devon page
http://www.cs.ncl.ac.uk/genuki/DEV/ It will give you good guidance. Another tip is to post your name interests and look-up requests on the DEVON page of Rootschat.
The numbers from the IGI are reference numbers of where the information is stored, so keep them.
The 1841 census information is pretty basic. Usually the ages are rounded down to the nearest 5 years. So if it states age 50, he could be anywhere between 50 & 54. Relationships to each other in a household are not recorded and place of birth is recorded as 'were you born in this county? answer yes or no.
Most parish records for anywhere are housed in the local History centre for that county, but LDS centres have copies as do some central family history centres in London. Someone can jump in here and list them as I'm not familiar with them.
There are no certificates for pre 1837, only entries in parish records.
Don't worry too much about not having your gggrandfather's birth record, if everything else tallies just proceed until such time as evidence presents itself, it usually does!
What makes you think he was illigitimate?
Happy Hunting. Before you know it you'll be helping someone else whose new!
Jane