There are a lot of questions here.
The IRS will drift, unless it is updated. There are many ways that this can occur. Since it appears your focus is on non-GPS ac, it must be updated manually.
For procedure designs, (FAA) the IRU drift rates are assumed to be 8nm/hour, but this far exceeds other issues.
While powered up, the IRS will continue to calc drift until updated, this includes the taxi time/speed, and ground time (as the IRS will continue to calc location based on the rotation of the Earth, ie you are sitting on the ground for an hour and the position difference) until an update is entered.
Location is always entered true.
As far as absolute system tolerances, this is an example from Honeywell.
