Gaboim: The “minhag” to give a bracelet by the engagement became widespread within my lifetime. I am very young. Not everything done by you or your circle is minhag yisroel.
THe reasons the engagement ring cannot be considered kiddushin are as follows.
1- no witnesses
2- a ring with a stone is passul according to all opinions
3- assukin b’oso inyan only applies if the ring was given min hastam. If it was given deliberately not b’toras kiddushin, it makes no differance.
BTW, I recall that the story with the rings in the school play was that R’ Moshe paskened that there was no need for a get of any kind.