To add: Yes, the order by Moscow is valid but the support from Ukraine is still cut.

No, that doesn't work. The support offered by A will be cut regardless of how many supports A receives itself.

This is a question we receive a lot, so it's a reasonable thought. But it's wrong!

Looking for some help. Lets say Unit A is supporting unit B to attack unit C. However, unit D is attacking unit A and cutting the support.

My question then, is could another unit (E) support unit A to hold, and thus prevent the support from being cut by Unit D?

To use the Diplomacy map, orders would like like the following:
Russian Army in Warsaw attacks Galicia
Russian Army in Ukraine supports attack on Galcia from Warsaw
Russian Army in Moscow supports Ukraine Hold (**is this a valid order**)
Turkey Army in Rumania attacks Ukraine
**is the Ukraine support cut? Or is it reinforced by the Moscow hold?**