Hellebores love a compost mulch but do not let mulch touch the stem of the plant as this could encourage disease. As for wood chips as a mulch, it all depends on how fine the chips are and what type of wood has been chipped up. I use a hard wood mulch of medium chopped branches and debris and my hellebores thrive. A finer mulch that allows rain and air to pass easily is the answer. Avoid using a cedar chip mulch as cedar repels water. As for your rhododendron, the yellow leaves are most often caused by poor drainage or by too much mulch piled on top of the shallow roots of the rhododendron. A soil that is not acid enough will also prevent a rhododendron from absorbing nutrients. Try using a plant food made for evergreens and rhododendrons this spring and see if you get a quick green up. Scrape away any  mulch more than one inch deep. Make sure there is not standing water after a rainfall. Rhodies are tough - I am hopeful yours will recover. Keep growing, Marianne Binetti