They come back sometimes - but not as full event. A few weeks ago there were some tournaments where you could get a small number of fragments from old fusions. But if you stopped at something like 50/100, don't bother about the fragments for that champ anymore, you are not going to complete it anyway.

You can still get the fusion champs from shards like any other champs.

Try not to do a row of events where you miss the champ, that's a big trap. Better skip one event completely and prepare for the next one: build 5* champs as food for champion training, build all the champs for Relickeeper fusion and save the fusion for champion chase (you cand do that more than once), don't use shards at random weekends and save them for summon rush, save silver for artifact enhancement.