The Oxford English Dictionary says under the entry for the word through that it "was "abbreviated thro'; in 15-18th c." and then gives the version without the apostrophe. The Century Dictionary has a separate entry "thro', thro"--presumably the version with an apostrophe is listed first because it was historically the older one. It is described as "A shorter form of through."I pronounce it the same way as through.
Now that you've asked, I'm wondering why anyone bothers spelling it thro'.
No doubt someone will explain soon
Your hypothesis may be correct, but I can't find anything to decide the matter one way or the other.I've got another theory. Does anyone know when the <gh> ceased to be pronounced? We know that it was> was originally a velar fricative [γ] and underwent different changes in the dialects, including [f] in cough and trough. The change or the loss seems to have taken place in the Early Modern English period (1450-1600). If there were still accents that still pronounced the <gh>, it might make sense to use the apostrophe to indicate the 'newer' pronunciation where the <gh> wasn't sounded. In the same vein, Shakespeare might have written lack'd to emphasise the fact that he wanted a monosyllabic pronunciation - and this would only make sense when a disyllabic pronunciation was still possible.
Yes, thru is not generally used in BrE, and I've occasionally seen thro or thro'. Not often, mind you.