Heya, beho!
The priest way of healing Loatheb is to start casting a Prayer of Healing when there is 2 seconds left of the debuff. This will heal every single member of your own group to full, including the main tank should he be in it. Then it varies. I personally cram off 2x CoH to heal 6 people to full, or 12 people to almost full. Then spend the next 17 seconds regaining mana (wanding), because you just blew a quarter of your manabar
For 10 man raids, this strategy allows me to solo heal the raid. For 25 man raids, two priests can
almost heal it all alone. This fight is seriously tweaked in favor of priests, as it allows us to use heavy burst healing, a thing we're excelling at. With only 1x CoH due to the cooldown, I can probably still soloheal the 10 man version if I use renew and shields (shields work while the debuff is up), but this will cost me my manapool very quickly as I won't get that 17 second break window regenerating mana.
Renew is not a good spell for this fight. It ticks for very little (800-100 depending on glyph), and only every 3 seconds, which means it will tick once during the 4 second window of healing. Still, if someone is falling behind, throwing a renew on them a few seconds before the debuff lift will delay their demise slightly.
Flash heal isn't too bad, but CoH is about 3 times better as it targets 6 people. While CoH won't heal enough alone, casting it twice very often do, as the raidwide damage on loatheb isn't that high and a flash heal will usually just overdo it. If the main tank is taking heavy damage, I may end up substituting one of the CoH's with a surge of light Flash Heal on him, but that's about it.
The "bouncy spell" (Prayer of Mending) is actually useless on this fight. You cannot control where it bounces, and it will bounce every time its holder takes damage. But as that will almost never happen when the debuff is down, it won't really heal anything.