bob-
q1- sounds like they may have sent you a wrong adaptor. Since they advertise 73.6mm... if they sent you a 70mm, then you should request an exchange, confirmation they sell the 74mm adaptor.
As a side note- the 10$ spacer link I sent you has both 70mm and 74 mm patterns... easy bandaid.
on Q2- all horn buttons on these cars are pressure fit.- held in place only by the contact made between the silver ring and the hub adaptor.
The chrome ring closes the circuit to make the horn button work. The button assembly can not/should not buttom out in the hub/adaptor because there is an electrical connector at the bottom of the horn button. If the bottom horn button electrical connector came in touch with the adaptor/ hub, it would close the circuit and your horn wold stay on until the contact was broken.
A foot test to see how the button assembly moves: if you hold the horn button by the ring with one hand, you will notice you can push in on the center of the button and it ( the black piece with the Porsche logo) will freely move down about 2mm. This is what closes the circuit and makes the horn sound in day to day operations.
i use my button regularly and have not experienced it ever sliding beyond the point where I originally pushed it in. It should be a snub fit.
im wondering if the adaptor pictured was over machined... the hole being a bit larger than expected...