What I meant re number of consecutive days is that you should be able to click on the first day, THEN edit the number of days field, and have the calendar update accordingly. You may need an update button to make this work. My experience was that I naturally do things in this order, and it was not working.
What I meant by suggesting (shift)-click is that either an ordinary click should extend dates selected, or if you prefer use shift-click instead. I think an ordinary click for extending the range of dates would be fine, as why else would you be clicking on two non-consecutive dates?