If the 'Mech shortage doesn't exist, why then do the House's build/use ten times the number of tanks then they do 'Mechs?
As for #3 given the number of upgrade kits that seem to have been made.
Because that's what the police, lower end nobles, and planetary guard are legally allowed to field in numbers, and I'm sure there's a cap, somewhere, too.
But, that's not the case everywhere. In the Periphery Field Manual, it's stated in the units section that most periphery worlds only field a battalion of hard armor assets, preferring to spend money on Mechs.
In fact, as a counter to the notion that the tank wall is possible, I would like to point out that most tanks from the TRos are manufactured in the same way as a lot of Mechs: On a particular world or a particular small number of plants and then shipped out to buyers. Yes, sometimes they're still in various states of assembly, if QwikCell is anything to go by.