vlog 5 splitting the table top & making strechers 

If you really have to use tapered mortices (which is a method originating in the USA, I believe), then the taper angle of the mortice must be precisely the same as the tenon, otherwise the seat (top) will split. All the antique Windsor chairs and Welsh stick chairs I have seen in the UK have cylindrical tenons, which are far easier to produce and much less prone to splitting. I have made dozens of chairs and stools in this way, and never had one split, and I always give the wedges a good bash. The master chaimaker, John Brown, always used cylindrical tenons as far as I know, which is a good reason to do likewise. Your top is rather thin for this method of construction. I would suggest over 40 mm. The ideal timber for seats (tops) is elm. It has interlocked grain which helps prevent splitting..Hope this helps.