yes.
ram expansion assert ramdis, but they also listen to ramdis and do not activate if they see it? in this way ram expansion can take priority.
the asic does asset ramdis to block all external and internal ram when asic ram is active?
I don't think any ram expansion listen to ramdis since they have to assert it during an access.
If you look at a 464/6128 schematic, you will see that RAMDIS only disable the buffer that drive the Z80 bus. The ram is read, but the result is just ignored.
On the 6128 PAL, RAMDIS also prevent write to the memory when asserted. This is missing on a 464 and requires some trick for the expansion to work.
RAMDIS is an input with a pulldown resistor. Extension should drive it in a wired or connection (hence the diode).
Basically, if you put multiple ram extension that maps to the same bank, all will be written and all will drive the bus during read. But this is not a problem as data should be the same.