It’s time to clear up some more misconceptions.
Erussin and Kiddushin are synonyms. They are the “harei at” part of the wedding, at which point the woman becomes an eishes ish, and is forbidden to all men except the man who betrothed (kiddushin or erussin) her. Not before. Repeat, not before. The nissuin part of the wedding is more complicated, as this is the part where the man takes the woman into his “reshus”. Some maintain this happens under the chuppah. Some maintain it happens as they enter the yichud room or even later. Separate discussion.
Modern usage of the terms has become sloppy and there are those who use the word “erussin” for an engagement party. This is misleading and untrue. There is no halachic bond between the couple at the L’chaim, vort, engagement party, or tenaim. None whatsoever. As I explained above, the tenaim create a contractual obligation between the parents or families or whoever obligates themselves to ensure the wedding takes place or money or dowry, or property, or whatever is transferred. Tenaim is a business contract, not a halachic one. Keeping both ends of this business deal of course becomes a halachic obligation, but is not realted to kiddushin or nissuin.
If there are no t’naim, that is, no business contract relating to the couple and monetary obligations,penalties for backing out and such (see my post above), then breaking the engagement (a secular term) is unfortunate to all, but not a halachic issue. An engaged couple means the man and woman decided to marry each other. They can change their minds and decide not to. The L’chaim, vort, or other parties were nice, but did not obligate anyone to do anything. They are celebrations and nothing more.
There is also a possibility, rarely if ever done nowadays to the best of my knowledge, of attaching certain conditions to the kiddushin, what is referred to as kiddushin al t’nai. This deserves a separate discussion and as I said, is not really practiced anymore.
Hope this helps.