1- No real difference, but the use of 'assuming' suggests that they might change their policy, though as they say 'as always', it sounds unlikely.
2- 'perform' suggests that you saw erveything, while 'performing' might indicate you saw part
3- b) suggests that you do this regularly- her staff could use this one as the accompany her on tour. a) & c) are pretty much the same as they are both now, so no real difference to me.
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