How can I tone down the heat in hot oriental mustard, without making it sweet/significantly altering flavor?
I came across this article.

It explains where mustard get its heat from:


  Mustard seeds come from the mustard plant, a member of the cabbage family. They contain two sulphur compounds, myrosin and sinigrin, as well as an enzyme, myrosinase. When the seeds are broken and water is added, the enzyme breaks down the sulphur compounds. The result is the sharp tasting oil that gives mustard its pungency, and helps explain why the name mustard comes from the Latin words mustum (much) and ardens (burning).


In addition, it recommends adding some flour to tone down its heat.


  In the case of prepared mustards, the reaction is toned down by using additives such as flour.