those marked illeg are illeg at time of birth but often a subsequent marriage will legitimise.
Illegs were usually known by father's name though by end of 19th century the English custom became more frequent.
It would appear that not all Parish clerks indicated illegs. My usual reading is that the absence of a father's name generally indicated illeg (which were quite common).
Manx spelling was somewhat phonetic until end of 19th c when after compulsory schooling most were literate. (Also as use of Manx was declining you could get some Parish clerks using an english translation of name - Mylevorrey to Morrison etc) thus some names are rendered differently in different years.
Also remember that the IGI is a somewhat poor transcription of another (tho generally reliable) transcription