First thing you might want to do is take out Lingering Ritual and replace it with something else, anything else. As a Templar healer you have better options than lingering ritual. I suggest Blessings of Protection (and it's morph Combat Prayer). Same magica cost, no cast time. Repentance (morph of Restoring Aura) is also a good choice if you can get it early. It uses corpses (any corpse will do even party members ) rather than magica and has the added benefit of restoring stamina to you and your party members, also no cast time.

Honor the dead seems like a good morph for the magica refund but Breath of Life hits multiple targets saving you from casting it multiple times so it's more efficient.

Take Healing Springs over Illustrious Healing. Illustrious Healing's 1 second duration increase doesn't make much of a difference over all but the magica restoration from Healing Springs does.

Resource management is always going to be something of an issue in the lower levels because most of your gear is likely throw together from quest rewards and what you can find. Plus you have a limited number of skill points to spend.

Things you can do to help with that would be to try and keep a Resto staff with you. Heavy attacks with the resto staff restore a bit of magica so throw a few heavy attacks out whenever your not casting. It also helps to use foods that boost your magica (and health whenever you can) and don't be afraid to use your potions.

To that end, consider taking up Alchemy and Provisioning. They're the two easiest crafts to level and makeing your own potions and food will save you no small amount of money (plus the passives that increase duration time are nice).

As for people running off on there own, there isn't much you can do about that. Sometimes people don't realize that mobs in group dungeons are harder than open world, delves and public dungeon mobs. Sometimes people panic, kite mobs and end up running way out of range of your heals. Sometimes people just don't care.

The only thing you really can do is ask them not to run off and explain that it's making it much more difficult to heal effectively when they do that. If they keep doing it after that then they're on their own, don't strain yourself trying to keep them alive. Hopefully they'll figure it out after that. If they don't or they get belligerent, well that's when you have to decide if it's worth staying with the group or not.

You will likely have resource management issues at lower levels. Skills like channeled focus and siphon spirit will help you later. If breath of life is costing you too much use healing springs.

Also before I forget dropped magica potions and eating food or drink that increases your max magica or magica regent also helps a lot and are relatively easy to come by