Paul, Well done on staying upright, seems to be more than I can do when sailing solo.
There was a thread on this subject a while ago, if you do a search on single-handed sailing with spinnaker you might find it.
Someone clever came up with a way of routing the retrieval line through a piece of bungy-cord which is attached to the tiller so that when the line is tensioned it keeps the boat going in a straight line. What I`ve found is that if you get the spinnaker up & down REALLY fast the boat doesn`t have time to change direction, this takes a bit of practice & a LOT of sweat, and prevents the kite going over the bows or snagging on anything else.
As for capsize prevention, I can give no useful advice here, since I went over 4 times last Saturday, twice on one downwind leg !

Cheers
Steve