validity of OJTI
The answer will depend on what approvals you have in place.
We have approval to carry out OJTI renewals in the simulator with pseudo trainees.
If you don't have approval to regain an OJTI endorsement on anything other than a real trainee in a live environment then the OJTI endorsement will lapse and it will need to be re-validated when you get a new trainee.
There can be additional complications in re-activating a lapsed endorsement and here in the UK our training plan has to contain a specific procedure for doing so.