Great to hear that MapleRead can open your book that can't be opened in Marvin!
Since I purchased both MapleRead SE (for iPad) and MapleRead CX (for iPhone) with Pro Upgrade many months ago, I don't remember the exact limit imposed on the free version CX. My advice is to go ahead to buy the upgrade, and everything should work.
Read the
FAQ on the developer's website regarding how to import from calibre or other sources.
Please post further questions regarding the usage of MapleRead in the
MapleRead Forum.