I was under this impression ...
1990's - Fleer, Pro Set/Classic, Donruss, playoff Skybox, Leaf, Pinnacle, Topps, Upper Deck.
2000-05 - Fleer, Donruss/Pinnacle, Playoff, leaf, Topps, Upper Deck
2006-09 - DonrussPlayoffLP (Leaf/pinnacle), topps, upper deck
2009 onwards - Topps Panini (donrussplayoff), Leaf
Basically with the panini purchasing the licensing from donruss playoff LP, Leaf was allowed to operate as a separate company. At least that's the impression I had.
As to UD returning, given their financial difficulties over the last few years (Konami, MLB Properties, MLB Players association, Kareem Adbul-Jabbar, Internal courtcase, MLB Properties round 2) and the fact that Fleer retro for basketball was delays for almost 18 months, I think they're lucky to be even have a contract with the NCAA/CLC. I honestly don't see them being allowed to even enter the negotiation room now that all of their dirty laundry has been aired.