Can hayfever / pollen cause anaphylaxis?

Anything is possible. It all depends on what you mean. Anaphylaxis, as defined by the involvement of more than one organ system, could surely cause anaphylaxis by causing asthmatic and urticarial/angioedema reactions. In terms of causing hypotension, i would assume that it is possible, but is rarely seen.
Not usually. Not usually but if the exposure is intense enough it is possible. Perhaps an allergist in healthtap can chime in with more information?