Annie. The first course of tiles should always "look up" slightly. ie they should have a lesser slope that the ones above them. Since there is no tile underneath them, this is done either with a double thickness first batten, or a high fascia board.
There should be no gap between first course and second course. Certainly no pugging with sand & cement.
Easy to do with your new tiling. Your problem is going to be how to marry the new into the old (side by side), since the old ones are looking down, and the new is a lesser pitch. Perhaps you can do something to lift a few of the old so they gradually run into the new, correct pitch. If not, you may have to be "creative" with a piece of lead to join the two sections Not easy, and not attractive.
I tend to have the opinion that wasps, as with mice etc, will always get in somewhere if they want to. Modern sarking felts do make it very difficult for wasps to get in now. Before breathable felts, old-type roof ventilators were always the best way in for insects. You don't have them now.