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Jewish Law prohibits the mixing of milk and meat in cooking. However, butter, a milk product, is a kitchen staple for most cooked emulsions (eg. pan-sauces, sauces béchamels, or roux-based sauces). While I use appropriate cooking fats for making non-dairy versions for meat dishes, this frequently doesn't work.
What is a non-dairy fat which may be used to as an emulsifier or roux ingredient?
Sauces like pan sauces and Roux-thickened sauces like bechamel or veloute are not traditionally finished in the same way.
Pan sauces, like for steak au poivre involve deglazing, reducing (still primarily water-based liquid,) and either reducing further with heavy cream, or adding cold butter directly and not heating anymore, so it stays emulsified.
Roux sauces like bechamel and Espagnole are made by adding hot liquid to hot roux in a pan. Though roux does a bit of emulsification— which is bonding fat based liquids with water based liquids— its primary purpose is still as a thickener. The starch cells in the flour explode, thickening the liquid. An uncooked version, beurre manie works the same way.
Roux itself simply involves toasting flour in fat, in approximately a 50/50 (by weight) mixture. From what I understand (I don't have my copy of On Food and Cooking around to double check,) the primary purpose of the fat in the roux is to keep flour from clumping up as the starch cells expand in hot water-based liquid, so the water and milk solids in the butter doesn't aid that goal in any way; in fact, in my experience, it causes some of the starch cells to explode prematurely, which makes butter roux a little more difficult to work with.
As the commenters pointed out, roux can be made with any fat— I've made it with many fats— so I'm guessing the trouble you're having with it might just be technique related. Since you said that you needed to add a water-based liquid to the roux to get it to work out, maybe your fat/flour ratio might not be right so it's too clumpy? As soon as you add a water-based liquid to a roux, you've stopped making a roux and started making a sauce with the roux.
Roux troubles aside, if you're looking for a parve emulsifier which adds a bit of richness— something to replace the role of butter in a pan sauce– letting your sauce cool a bit, whisking egg yolks into it and heating it to around 175F would work really well. If you don't have a thermometer, look on google images/youtube or whatever how to determine if a sauce is 'napé'... something which I think you have to see, rather than have it explained to you. Keep in mind that heating it to anything much above 180F and you'll have scrambled eggs. This would be my go-to for something like a kosher steak au poivre pan sauce.
You could always go the food nouveau route and use a glace— a heavily reduced stock. Boxed stock won't do, but if it's for a special meal, a pressure cooker or crock pot stock will work beautifully.
You can use most margarines, which are pareve. Most margarines contain lecithin to emulsify the (vegetable) oils with water to yield a butter-like texture. The lecithin should serve the same purpose as the emulsifiers found in butter.
Kosher cooks have long used margarine as a general substitute for butter. I have not tested it in a pan sauce, but I would expect it to work.
