Hi paisley,
In terms of quality and value … I wouldn’t hesitate to buy any mattress that SleepEz made. Lew was correct though that these mattresses are only available locally because they can’t be shipped by courier. This may change over the next couple of months though 
I believe that the bottom two layers are not separate layers even if they are separate ILD’s so only the top layer could be exchanged but of course it’s always best to talk with them directly to make sure that this is still the case.
Yes … SleepEz does this as well (see the bottom of the page here)
Wool is a great material but it is nothing like latex or other foam materials and it has it’s own feel and pros and cons. You can read a bit more in post #8 here which also links to some good sources of wool toppers which are very knowledgeable about their products if you talk with them.
Phoenix