Is it by any chance normal for A series to run 17-18 volts???
No it is not normal to run those voltages, & it is not normal for a qualified mechanic to be unable to put his finger on the problem in a flash.
The original regulators are adjustable & the design of them must allow voltage to rise as they age or become worn as my regulator was puting out enough voltage to boil the battery dry in a relatively short time also, & this was in 1980.
In my case I fixed the problem by fitting a brand new one.
I have seen a number of Datsun 1000 & 1200 cars with a solid state replacement regulator, so obviously these things must now exist & are probably the way to go.
Full details on how to adjust the original regulators in 1000's & 1200's are contained in the Gregories workshop manuals [No. 87] & I highly recomend that you get one, although I didn't see any when I had a look through eBay 
& eBay 
a few moments ago.
One of these books will prove to be a godsend if you wish to try fixing it by adjustment