Is only the main applicant of a work visa allowed to apply for ILR?
A PBS Dependent requires 5 years residence before they can apply for ILR.
In the second scenario for which you have responded that the ILR will not be reset, can the person on Tier 1 Exceptional Talent Partner visa apply for ILR after he/she has completed a cumulative of 3 years combined (for e.g., 2 years on Tier 2 General and 1 year on Tier 1 Exceptional Talent Partner)?
No. The 3 year concession is ONLY for the main PBS visa holder. PBS dependents need 5 years as a PBS Dependent before they can apply for ILR. You cannot combine Tier 2 main visa holder time with Tier 1 dependent visa to make up the time for ILR.