As David indicates rotas are one practical solution.
Although if you are not using rotas for any other input sequence, it may be confusing to end users.
The optimal solution may be rotas.
As to how to form the rotas, depends on how many other characters take the dot below diacritic. Is it just n or is it used with other base characters?
If not only n, do other characters take sentence final positions?
If only n, what are the relative frequencies of:
If I were developing the layout I would also consider if the period key was the most suitable key to se as a compose key.
I know it is a common approach a certain class of keyboards to use the period key to input the dot above or dot below character but overloading common, frequently used chacaters can be problematic.
So possible solutions:
define an interupt key that could nbe typed between ‘n’ and ‘.’ To produce fullstop instead of ṇ. This approach was used in some Vietnamese keyboards. Ie ‘o’ + ‘.’ = ọ and ‘o’ + ‘’ + ‘.’ = o.
rethink design. Is the period key the best or only option for the dot below diacritic?
There are probably other approaches. But these are the three that come to mind straight away.