The wingtip pieces that are mentioned are shown on the plan at each end of the stab and the elevators. They are simply caps to cover up the end grain of the leading and trailing edges.

I was going to suggest that you use the pins beside the wood rather than through it, but I looked closely at the pics and I see that you are doing that already. It helps you avoid splitting the thinner pieces.

So far it's looking good, just continue to be patient and work carefully and the plane will turn out great. And don't hesitate to bring the questions here. This a great site for info.