I wouldn't feel so sure about using a closantel based in place of a triclabendazole based unless you are taking into account that you will be missing the young fluke and thus may need more treatments of closantel based in that year. Alternating will lower the chances of resistance forming though. You could alternate by using closantel based at certain times in the year when less likely to have young fluke. I.e at turn out.
Nope TCBZ is the only one that will get all stages. TCBZ resistance is in the UK, but it isn't everywhere. I know of at least one farm that has been rigorously confirmed to have resistance, and many others that have strongly suspected. But SiN is right, your vet will know the situation in the area. If you are worried about resistance in your flock do a faecal egg count reduction test (get an egg count for fluke done before and 2-3 weeks after treatment).
HTH
Dans
edited because I sent too soon