Hi Tony,
In our latest update we have deployed a fix to this problem, you should find that you can now add payment to an invoice held within a locked transaction period.
As far as deleting transactions within a locked period is concerned, we purposefully added this functionality to remove the possibility of records being removed by mistake or in order to alter records.
As you mentioned that this is a routine task for accountants, we can certainly look at making a change to this should the suggestion receive a number of votes
I agree. It is a big problem when reallocating invoices.