Interpretation: on the contrary, I welcome the modern concerns for interpretation, in particular the question of interpreting different styles of music in different ways, as opposed to the old schools, where a master would teach a single set of rules to interpret the entire literature (but where different masters would have different sets of rules, each claiming that his rules were the only valid ones).
Expression: I don't think it to be out of place in baroque music. For example, I can't imagine the Fantasia in g (BWV 542) being played without expression, and even passion. Bach is much more than counterpoint and fugues! So I don't see expression as being absent from baroque music - rather, it is the interpretative means of expression that are very different from those of romantic music.
Koopman's jumpiness: I, too, feel he's sometimes over the top. Keep in mind, though, that one of the origins of music - especially baroque music - is dancing, so having things "jump" (figuratively speaking) is not necessarily a bad thing in itself.
Legato in Bach: yes, it is not stylistically correct. But it can be very beautiful. Despite what I said above about appreciating the progress in our musicological understanding, I'm still quite fond of the old Bach LPs by Édouard Commette: yes, it's the old French school, with its legato "by default", its absence of agogical devices, its repeated notes played at precisely half their written values, etc., etc. Yet, I find a quality in Commette's playing, which really sings and breathes - and you can clearly hear all the voices even in a fugue. (Marcel Dupré's Bach playing, on the other hand, never did it for me: although he obeys what "on paper" appears to be exactly the same rules, his playing strikes me as dull most of the time.)
Pianistic approach: since you are more attracted to baroque music, avoiding a pianistic approach is of course understandable. But then, I'm curious: do you play the harpsichord?
Urtext: I totally agree about that point.