1) No, the trans takes Toyota type WS fluid in all likelyhood. It may use type T-IV but I think it is too new, either way DON"T use Dexron in the trans.
2) I would just replace what you can suck out from the reservoir and call it good.
3) Most people never replace the fluid at all so just doing what is in the res. every few years should be just fine, if in doubt go to smg.toyotapartsandservice.com and look up the maintenance intervals for your van.
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