I will give the usual disclaimer that I am far from being a Tolkien scholar on the level of others.
However, it's my understanding that
Isengard was simply a translation, consistent with other place names in Rohan, from the Sindarin name
Angrenost.
And what's so complicated about
Helm's Deep? As explained in
Unfinished Tales, it was
Quote:
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A deep gorge near the north-western end of Ered Nimrais, at the entrance to which was built the Hornburg; named after King Helm, who took refuge from his enemies there in the Long Winter of Third Age 2758-9.
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With the myriad numbers of place names in various countries and languages, coincidence alone can easily account for similarities between 'real' locales and those in the mythos.