To Answer your questions as you asked them:
That is correct, MetLife will no longer accept ANY disability applications for individual coverage as of September 1, 2016. In fact any applications that are in underwriting if they are not completed and put in force by 10-31-2016 then applications will be canceled.
Guardian is a fine company and perfectly fine contract to go with, you should have no reservations about that policy.
For some specialties that is correct, they only have M/N for 24 months. A number of people will tell you that is not good enough but my suggestion is to ask them to refer you to just 2 of their clients who are physicians and have had a M/N claim for greater than 24 months. I will tell you I have been in this business for 23 years and have not only not personally experienced it with any of my clients but I don't have any colleagues that have either.
The Transitional definition from Principal is pretty simple, they will pay 100% of the benefit until post disability income from your next occupation plus the benefit from the policy start to exceed your pre-disability income. The reality is if you make $100k then that is not a great policy definition in my opinion but if you make $600,000 then even if you have the $17k max (that Principal will issue at the moment) monthly benefit which turns int0 $204k then the post-disability income would need to be $396k before your benefits would be reduced....that is a pretty good second job. The reality is the more you make the better this definition is since it becomes harder to find that 2nd (post disability) job that is high paying.
Depends on why you were rejected as to whether another carrier would accept you. Make sure you tell your rep exactly what the rejection was if you are using a different rep. As an example, one could get rejected for being a diabetic by one carrier but accepted by another. However if you have had a valve replacement you probably would be rejected by all carriers.
Hope that helps.