Just skip the clapping/cowbell/tambourine parts. They're worth some points but really not very many, and if you were really trying to maximize points you wouldn't be doing this in the first place. Skipping them does NOT bring you any closer to failing, and this is precisely to facilitate what you're trying to do. Those parts are there, as you say, to give the dedicated vocalist something to do during instrumental parts, but you already have something to do, so do that :-)