According to responses to this question, there was a dichotomy between northern -s and southern -th in Middle English.
What I am wondering is:
When and how was the vowel dropped in the ending, such as "makes" /meiks/? I can imagine a number of scenarios, including:
Some northern (-es) dialects dropped the vowel; some southern dialects (-eth) dropped the vowel. The going standard had been a southern dialect with the vowel retained, but a northern dialect with the vowel dropped gained predominance.
The vowel was never pronounced in northern dialects, even right when they borrowed the ending from Norse. It was never dropped in -eth dialects. (I find this unlikely, since it seems that the vowel would not be written if it was never present.)
The vowel was dropped contemporaneously in northern and southern dialects of English. However, whenever we read old texts, we are only aware of an era before the dropping of the vowel, so we (sometimes inaccurately) always pronounce the ending /eþ/.
What were the rules on voicing of the consonant in third-person singular -eth in Old/Middle/Early Modern English?
Did the rules for retention/deletion of the vowel in the ending always apply equally to "thou" -est forms as long as they were productive?
Anything worthy of note regarding the presence of the vowel in past tense second-person endings?