Alpha strike only matters vs. beam PD. Against AMMs total magazine depth is more important than alpha strike (assuming good sensor coverage).
I disagree.
If you fire a single massed volley, the missiles pass through the AMM interception window once.
The target can only fire AMMs for roughly (AMM range / (Missile speed - Target speed)).
If the missiles come in multiple smaller volleys then the duration is extended by roughly (number of volleys * reload time).
At minimum the target can fire AMMs for longer (I'm assuming the target uses normal AMM launchers not box AMM launchers). The phrase defeat in detail springs to mind, although it's not an exact fit.
Against shields DPS is more important than either.
I disagree. If you let the target survive the first strike then you give them time to recharge some or all of their shields, which means you must spend warhead strength bringing them down again. This is sub-optimal.
A missile strike should account for shield strength when calculating volley size. The objective is to reduce the shields to 0 _and_ do enough damage to the hull to destroy the target or render it mission killed. If you have no intel, trying to calculate this is a gamble. (If you forget to account for shields then blame the Tac officer.)
If you haven't got any intel, firing a single missile at an enemy will at minimum tell you what ECM they have, and they might reveal if they have AMMs or what PD they have.
In both the latter cases sustained fire from reloadable launchers + magazines gives you more penetration per tonnage because you get much greater magazine depth per tonnage. Spending 30 hull space on box launchers gives you 200 MSP worth of missiles that you can fire all at once. Spending the same 30 hull spaces on full size launchers + magazines at a 1:1 launcher:magazine ratio gives you around 300 MSP worth of missiles, which you can fire in about 20 launch cycles.
I don't want to start an argument about the precise magazine specs, but you can only approach a total of 300 MSP if your magazine has no extra HTK and very high efficiency tech. In most cases I grant that you'll probably have more MSP to hand, but it seems unlikely to be 50% more.
Against an enemy that can take 10 MSP worth of your missiles on his beam PD + shields per launch cycle, yes, box launchers mean you put 190 MSP on target vs. 100 MSP for magazine launchers. But against an enemy that has sufficient magazine depth and sensor coverage to kill 150 MSP with AMMs, you get 50 MSP on target with box launchers and 150 on target with magazine launchers.
If I've understood your scenario correctly (and it's late so forgive me if I missed something), if you fire 15 MSP per volley at a target which can destroy 10 MSP with beam PD, the target only needs to use AMMs to destroy 5 MSP per volley.
After 20 volleys the target has destroyed 100 MSP with AMMs and 200 with beam PD, taking no damage. It still has AMMs to counter another 50 MSP of missiles.