how about trying cassia first to see if that gives you the conditioning you want? I think if you apply it immediately after mixing there shouldn't be much dye release. If it does happen to stain a noticeable yellow, it should fade within a few weeks.
With the henna, just test strand as much as possible trying different times and different cassia/henna ratio. On blonde hair it can go coppery, red to auburn.
You could try a ammonia/peroxide free deposit only temporary color to see how you look with red before going permanent with henna. or if you have a wig and extensions store/beauty supply store around try on red hair there. I don't think anything else can really prepare you to know how a full head of red hair will look on you.
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