Thank you for reaching out to Ask Extension. It is difficult to give a definitive diagnosis from photos, but it is likely that your deodar cedars have Cedar needle blight (sometimes needle cast), which is caused by a fungus called Sirococcus conigenus. This organism is positively correlated with a pathogenic nematode called Bursaphelenchus xylophilus, which is associated with Pine Wilt, also called Pine Wood Nematode (this pathogen is not exclusive to pines despite the name). 

There are various names that can be used to describe the symptoms that you are describing. It is possible that all of the arborists were referring to the same condition. The cycle of these pathogens is very fast, and a healthy tree can look great and within a year be dead. Unfortunately, there is nothing approved for these pathogens, but as you can see in the second link below there are some fungicides that can be used to treat the fungus associated with the needle blight. Arbor Kelp is considered a fertilizer, which can stimulate growth, and is not advisable when trees are losing their needles. 

For additional information about these pathogens, you can take a look at these resources: Pine (Pinus spp.)-Pine Wilt | Pacific Northwest Pest Management Handbooks and Cedar (Cedrus spp.)-Needle Blight | Pacific Northwest Pest Management Handbooks

That sounds like Phytophthora, which is a water borne pathogen that causes dieback and root rot. It is sometimes called a fungus, but it is actually more closely related to algae. Unfortunately, the fungicide will not help with Phytophthora. Once it is established in an area, it is very hard to control. There are some cultural practices that can help prevent the spread - cleaning your shoes with Lysol or Formula 409 after walking in the area and placing western redcedar chips throughout the area. It is a good idea to try to confirm if Phytophthora is present. There are a few resources available that can help. You can send in a sample to the OSU Plant Clinic and/or you can purchase testing strips. There is information for both below. 

OSU Plant Clinic - Phytophthora testing Services and Fees | College of Agricultural Sciences

A soil test might give you the best chance of identifying the pathogen. The other test available requires taking a piece of the root. 

Phytophthora Testing Strips - Agdia - Agdia ImmunoStrip for Phytophthora (Phyt)

Have you recently planted any new plants in the last couple of years? Phytophthora is often spread with newly planted plants. Have any other plants in your yard shown a decline? You would likely see nearby plants showing some signs of decline, such as a rhododendron. 

It is possible that it could be a different pathogen. Armillaria can have similar symptoms - dieback, yellowing needles, and root rot. With Armillaria, you might see mushrooms at the base of the tree. Here is additional information on Armillaria: Armillaria Root Disease.

Please feel free to send follow-up questions.