For an extraordinary look at the professional military culture extant in the US today, read this Abu Muqawama post and comment thread in its entirety. Pack a lunch (or a substantial snack and a drink), the comment thread covers six days of well-considered point and rebuttal by a broad collection of (mostly) very experienced (and also mostly) currently - or very recently - serving officer and senior enlisted Soldiers and Marines from both the Afghanistan and Iraq campaigns.
For my own part, I would council avoidance of using the Merchant Marine as an ancillary force model, better I think to adapt the US Coast Guards dual civil authority arrangement to an infantry soldier's environment.
As to the training quandary I seriously suggest that this might be better dealt with by revamping the basic terms of military enlistment. By making the basic period of enlistment a full 6 years of active duty with the award of a university AA/AS degree as a result of the training and professional education a service member receives (and the realistic possibility of a bachelors degree being achievable on his/her own time) as well as a bachelors degree being the result of a re-enlistment for an additional 6 years. The US military gains the added time in service to achieve the expanded education and training modern military mission variety and equipment mix demands while enlistees recieve the advanced education and experience they hope to attain from military service. At the end of the 12 years the option to continue career military, transfer to the ancillary force or pursue civilian opportunities will have to be chosen between - by both parties (there being no guarantee that the service will require quite as many relatively age'd infantry in the strategic mix).
There are undoubtedly numerous good reasons for not doing anything like this - but let me be the first to point out that how much more difficult it is to lead men and women who are at least as well educated and experienced as yourself will most pointedly not be acknowledged in the decision-making process. The US military services have historically been a professional organisation for the officers after all. For all the undoubted changes in the various branch's in recent decades, that hasn't been one of them I'm certain.
And yes, I was junior enlisted once upon a time; does it show? :)