easy things first
Re things manx - suggest that any serious fam historian invest in the 2 sheet public rights of way may - why this one (a) it is based on the 1880's 6 in/mile updateed but keeping the detail which includes most farm names etc; have a look at my web site and buy eg one of the better guides (eg Stenning's Isle of Man is easily obtainable on 2nd hand for about £5)
Note that few civil records prior to 1877 and thus IGI is based on transcribed parish registers which are of christenings (generally within a week or days of birth but not always) - those IGI records that are based on parish records are generally reliable transcipts of what are in the records - those of patron submitted info (eg sealings for dead) are NOT
Birth dates were of course recorded by family (eg in bibles) or may be obtaianbale from info supplied eg on joining the army, applying for a pension.
The IGI on fiche has recently been updated - that on-line is always up to date; the previous fiche I think dates from early 1990's
Names sometimes are changed (eg a John James might be under John but used James ) - spelling of Manx names was not standardised and you may need some immagination to guess how non-Manx rendered them