You can download as many times as possible, the acsm, no 3 times limit.
Does the website impose such limit ?
Also if you did not authorize your rmsdk client(a.k.a ereader) without an adobe id then you could run into problems restoring your ebooks if you lost your ebooks!
And ADE does not encrypt the books. It only reads by decrypting.
only owner of book has unencrypted copy
book is encrypted and stored in a mediaserver(cdn may be)
the fulfillment server serves the book to you, and also gives you a key to decrypt it. this key also is encrypted.
the drm crackers can explain in full detail on how decryption happens
for more info google for search content server manuals!
always on drm for ebooks seriously ? don't even know if its worth it.
on the other hand epub3 which has interactive or streaming video content may require connectivity.
browser based eBook reader which reads books from cloud and serves you page by page, caches the next and previous pages and also does some data mining and quickly serves you the pages may be the future if everything moves to cloud..
your books always online like gmail or this very own FORUM.