my point was you need to be careful which intermediates/LG you discuss because they change depending on whether the reaction is under acidic or basic conditions.
for clarification C-NH3+ refers to the tetrahedral amide intermediate with sufficient proton transfers to give nitrogen - still attached to the tetrahedral amide intermediate - with 3 protons (thus nitrogen has a positive charge). sorry if I was confusing.
I've drawn the reaction under basic, neutral, and acidic conditions. Under basic conditions, there's no way to get NH3 as a leaving group. You'd have to either have way too many charges on the intermediate, or even if everything else was protonated, you'd have a positively charged intermediate in a basic mechanism. This won't happen. The base will remove that proton on the NH3 grop way faster than NH3 will leave as a leaving group.
The question was concerning hydrolysis under basic conditions. I recognize the reference you've provided... but I'm very uncomfortable proposing RNH3+ as an intermediate in a mechanism under basic conditions... The only reason it works here is the protonation states of the rest of the molecule result in the molecule having an overall net negative or net neutral charge. For undergrads just starting out, this may be a bit confusing. I would caution that some might try to have a molecule with a net positive charge... This is the species I contend will not form under basic conditions. Even still, I contend that deprotonation of the RNH3+ is MUCH more favorable than the ROH in that intermediate (what I guess they're calling B2). The pKa of RNH3+ is MUCH lower than ROH and the RNH3+ should be deprotonated MUCH faster, imho.
Most undergrad text would probably draw the mechanism as I've drawn it below. op should check with the instructor for what the correct mechanism will be in an exam situation.
You might also be interested in this discussion on whether reaction mechanisms can even be proven: http://www.chemistry-blog.com/2009/05/13/survivor-mechanisms/