Apart from a few misunderstood points, you have got the picture right. Before answering your question let me clear these first:
As a discipline it shall be your job to maximum assist at tank healing. For that I usually recommend keeping a shield up as you mentioned and to Penance when needed, since that is your big burst heal. In emergencies you can use flash heal as well to top off them. Barely if ever greater heal, since it is a big heal, but it is very slow and ineffecrive to be used by a disc. I do not recommend it, only in clutch moments, and if possible combined with inner focus.
Now the shield on the tank is usually a good option to apply on the start, since they will definitely take damage. Keep in mind tho that it will most likely be the first shield to pop and usually alone, immediately causing your rapture to go on cd. If you can allow this mana sacrifise, then do go for the preshield. If not, then try harmonising the shields with a huge rade-wide damage (eg. Infest) to get the most mana regen out of rapture.
Do not use Holy Nova on the tank, as it only heals your own group, and only the ones in range for it. It can be used effectively inside of frostmourne only, if your group has taken moderate damage already (they drop to 70% hp). Other than these grouped up scenarios, do not use this expensive heal.
Now about the renew usage. I usually cast renew on the tanks if I have a free gcd to use. On the pull I am usually busy bubbling the whole raid, so I would recommend to waste a few ticks of renew and apply it before the start. You can also apply a pom on the tank to keep it jumping on the raid right on the pull. Also your shields absorb around 10k damage, so they usually break on the first aa from the boss, this means if you apply renew right on the pull, you wont waste off any ticks. Do this if the fight is either not aoe heavy qnd you only have to focus on the tank or of you have already covered up the raid with shields.
During the raid follow the rule if you have a free gcd, apply renew on the tank. Keeping it up all the time is not your priority as you are not a hot healer :)
This reply grew a bit long, but I hope I answered all your questions, if I left anythig unanswered, please let me know.
I typed this from my phone so sorry if I did many mistypes.