I'm not certain on the current drawn by these specific glow plugs, but most glows draw more than 10A when cold. That will blow the fuse in most common meters, and most people don't have an external shunt available.

However you can use this product if you happen to have one around. They are handy for all sorts of moderate power DC testing and surprisingly accurate for their low cost.

But if you don't have something like that, the size of the spark is something people have been doing for years. More current = more spark, and a plug that measures fine for cold resistance may expose connection issues as it heats up, resulting in a weak spark for a weak glow plug.

However with the readings you've shared, it's starting to look like a failure of the monitoring system in your glow plug relay. As long as the car starts well, it's more of a cosmetic issue at this point. I'm guessing it's fully functional - I currently have 1 dead glow plug on my 300D and any time it's much below freezing that cylinder misfires and puffs white smoke until it warms up.

As long as the glows heat up and then switch off, that's all you need out of it. Glow plugs are a simple system. In fact my Mercedes is the only diesel I own for which you don't just hold in a glow plug switch for 30 seconds, then crank.