Re: to commence = is to be commence = will commence
As a Brit, Procurement Advisor and mentor, but not a teacher, I would read your text as meaning:
Delivery shall (i.e. must) commence in six to eight months and shall (i.e. must) be completed within twelve to sixteen months.
If not, I would regard them as being in breach of the Agreement and liable to appropriate Contract remedies.
I would regard your sentence as being written in "technical" English (a sort of "shorthand" English, where words not vital to the understanding of the text are ommitted) rather than strict grammatical English.
[The] Delivery [of the Goods is] to commence in six to eight months and [is]to be completed in twelve to sixteen months.