No, unfortunately only the UKC's income can be considered for the visa.
Did he make over £18,600 in those 4 months? If so, it's okay that it was for less than 6 months (so long as he has a job offer in the UK meeting the requirement).
The USC's income can be considered for the NEXT visa that is applied for from within the UK though.
No unfortunately not, though if he applied via category B if he still did not earn the £18,600 over 6 months I assume it's okay if they can predict that his per annum would be well over this amount?
Due to the whole MIR amount in the U.K this is our long route of moving to the U.K via him coming here first.
In terms of priority how long does it usually take, its recommended by many but what is the rough time span ..has anyone received a decision quicker than 4 weeks?
I believe when applying it asks on the form if anyone is traveling with you and you list your spouse. If the U.K spouse moves back a week or two earlier and the U.S spouse travels on own to U.K is this okay or does the U.K spouse need to be there to show employment letter.