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It is safe to use it, but some people feel that the fact that cedar decomposes so slowly, that it can harm the fertility of the soil. A bark mulch or hardwood mulch will also work.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberry-pruning.htm

Winter dieback is normal, and plants should regrow in spring.

Use an organic fertilizer, such as compost, which can be applied during planting. You can also mix some with water and apply during watering about once a month, although this isn't a requirement.

It is most likely plant rust. This article will help:
https://www.gardeningknowhow.com/plant-problems/disease/learn-about-plant-rust-disease-and-rust-treatment.htm

If they are growing well, you should let them be as they are.

Some varieties of blackberries are blended with others to create a blackberry that is thornless as the plant bought, but that will revert to a thorny blackberry if propagated. These sucker are a form of propagation (though not intentional), so they grow with thorns. It would be best to cut out the shoots that have the thorns.