I don't think any ereader does "tabs".
As suggested, the PB seems to do multitasking well.
The Kobo or Kindle should also do well, in the sense that your place in a book is remembered, and when you re-open the book (from the home screen) you will be exactly where you were before. Even though the book isn't
open the whole time, it should come to the same thing.
Page Numbers:
On the Kindle, there is a GOTO menu, where you can type in page numbers to go to -- but only if the book has an
APNX. Many books purchased from Amazon have an APNX, but sideloaded books will probably have to have one generated. It would depend on how good your source info was. PDFs automatically have page numbers, and entirely accurate ones.
Any other reader will probably be ADE-based, and will lie to you by saying (assuming there isn't a
Page-map or
PageList inside the book) that every 1024 characters (compressed) in the book's text is a page -- even though it almost definitely isn't. I guess the idea is, most people won't notice the difference if you just toss them a wild (rough) guess -- of course, they happen to be right.
I don't know how Kobo/B&N handle accurate Page Numbers in store-bought books.
calibre can convert EPUBs to AZW3 for the Kindle, if necessary (or vice versa).