The best way to manually manage your signature is to update the activity with the incorrect signature and use the Save/Lock function under Actiivty Details/Advanced tab to correct it. It will then propagate that change based on your Signature Decay Method forward.
Adding very old activities and deleting them should result in the same final signature unless there have been other changes like changing the signature decay method, time constants or the starting training load.
Recalculating your progression is only recommended when you wish to undo all your changes and start over again. Read the instructions carefully.