I think you have it more or less correct, but I'd strongly suggest you either get a VFD designed for single-phase input or, as Indexer says, get one that is specifically rated for both.
When I was in the market for one, one place I looked told me I could do as you suggest -- run a VFD designed for 3-phase input on single phase, and de-rate it. While that was probably true, I decided to skip all the kuldging around and get one specifically designed for single-phase input. There's no real reason not to, if that's what you need. There are plenty available, if not at one dealer then at another.
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