Hopefully, with the release of Episode II and III we will have some decent answers to these questions.

A.  The Jedi master that trained Obi-Wan Kenobi.

When in ESB Luke Skywalker is told by Obi-Wan's ghost to seek out Yoda, the Jedi master that trained him, it seems to be in conflict with Episode I. TPM shows the audience that Qui-Gon Jinn is actually Obi-Wan's teacher. In Episode II, however, we see Yoda training a group of children. This supports the fact that the training of a Jedi is more of a collective effort at the begining, before a master takes on a Padawan learner.

When Yoda called Dooku "my old Padawan", this may have been what he meant. Of course he may have also meant that Dooku was his apprentice.

B.  How come Owen Lars doesn't recognize the droids he buys in ANH as Artoo and Threepio from AOTC?

Truly an interesting question seeing as he had already owned Threepio once.