Different marriage scenario - Need help!

If by "UK Border Agency hotline" you mean Worldbridge, then yes, they are complete rubbish and don't know a thing.  This is a well-known problem, yet nothing ever seems to be done to change it.  

You definitely cannot switch to FLR(M) from a visitor visa (that's the stamp that he would get in his passport at the border).  I will try to dig up the rule for you, or if Weby happens along she has it at her fingertips.  But it can't be done, and if he tries he'll be refused.  Best thing is to apply from the US using the premium processing.  

ETA: Here is the guidance (and props to Weby, I took it from one of her posts)

As stated above all of the relevant provisions must be referred to when considering applications for leave to remain in this category, but in general caseworkers need to be satisfied that:

-the applicant has limited leave to remain in the United Kingdom in accordance with the Immigration Rules, other than limited leave to enter for 6 months or less (unless the leave in question is limited leave to enter as a fiancé(e), in which case 6 months is acceptable) and has contracted a valid marriage, which is recognised in this country, to a person who is present and settled here;

-the marriage is subsisting and that the couple intend living together permanently as husband and wife.

The "no switching" into marriage provision prevents those given limited leave to enter the United Kingdom for 6 months or less, e.g. visitors and short term students, from switching into the marriage category. It does not apply to those given leave to enter the United Kingdom for 6 months as a fiancé(e). Neither does it apply to those who have been granted an extension of stay in the United Kingdom of six months or less at the end of their initial period of leave.

Here's the source document: http://www.ukba.homeoffice.gov.uk/sitecontent/documents/policyandlaw/IDIs/idischapter8/section1/section1.pdf?view=Binary

Complete rubbish, huh? Why am I not surprised.

Thank you for the info.

Geez, and I kept thinking the difficult part of our relationship would be the year apart; and worrying about him in Afghanistan.  I had no idea this could get so complicated.

Again, many thanks!

The good news is that it probably seems a lot more complicated than it actually is. The bad news is that it's not exactly the fastest process in the world.    Best of luck with the visa.