merging alpha masks

I am not sure how the merged alpha channel will help you and I am pretty sure if you applied that combined alpha channel as a layer mask on the merged images that you would not get the same result as a Merge operation (Michael already mentioned this above). In the layer stack, each Layer Mask operates wtih the existing layer and the next lower layer (with all other layer settings) to determine the Composite image to present to the next higher  Layer and Layer Mask.  The masks are not directly working in a cascade fashion with each other directly from a conceptual point of view (IMHO of course).  That is why preserve only keeps the bottom Layer Mask. All the other masks were merged into their respective Layers druing the merge process (except the last one).

That given, if you want to create a composite mask for some purpose that I do not understand (very likely) then here is how you can do it:

- Make sure that selections are cleared

- On each Layer, add the Layer Mask to the selection

- After all Layer Masks have been added to the selection, either a) save selection as a) Channel Mask or b) create a new Layer Mask on its own independent layer from the selection.

At this point you have the combined mask as you requested either in the channels panel or as an independent layer mask.   You can do the merge without preserve set and still have a combined mask as you requested to be used whereever.

Hope this is helpful and if I just don't  understand your request I apologize for taking up your time.   If I am off base, it may be helpful to describe what you want to achieve with your image as opposed to just what operations you want to execute.  I sense other forum members don't quite understand the end result you want to achieve.  I am sure you will get lots of suggestions once forum members understand the end image objectives.  Good luck.

I think what I would do (and I'm no expert) is to use actions to bring the individual masks to the non-mask area, by cutting and pasting them from the alpha to the proper layers. Change the layer type of either multiply or lighten, depending on your setup, and then copy merge all the masks together, and paste that into whatever layer's alpha you need. All of this should be recorded in an action.

I'm partial to using actions. There might be a fast way to do it with scripts.

I am trying to understand what you want to do. There are couple of ways to combine the masks into one. But I'm not sure how you want to merge the layers. Unless you apply each layer's mask as you merge( in which case you don't need to remask ) the final merged layer will not look the same as the 10 masked layers even with the new combined mask.