for non-interruption as a shadow priest you have to look at exactly what you are doing.
pretty much your number one is the shield. nothing compares to that. if you are soloing, then by the time the shield runs out, you should be wanding anyway, which doesn't get interrupted.
if you are PvE grouping, add to that the reducing threat talents so they don't come towards you in the first place. so that is shadow affinity, silent resolve (both of this give 44% damage threat reduction) and improved fade. unbreakable will would likely be handy as well. martyrdom is also of some limited use, but they have removed the focused casting spell so it's certainly not as useful as it once was. (it is 8s of uninterruptability if you get critted... i don't really rate it because if i get critted, i'm half dead, and the last thing i'm thinking is "how can i continue healing?" - can be useful for a shadow priest, but i noticed that it mostly procced when shield was up anyway)
it is important to note that healing focus is only for healing spells. as a shadow priest, you will not be using these when soloing. you will only be using them if you are healing in a group. is it worth taking? sure. if you're happy to spend a couple of points there rather than in something that will increase your dps ability. if you are looking for an all-round instance build, it may be worthwhile, but as a shadow priest i was mostly thinking about the damage above all else. realistically in a group setting, the tank should have aggro off you before shield runs out, and if they don't you're probably screwed either way.
as to your mana pool comments, i don't quite understand what you're saying. vampiric embrace restores health, not mana. the mana pool-increasing talent is mental strength and you have to have 20 points in disc to start investing in it. so it would mean you can't have both that and shadowform. a shadow priest will not have mana pool problems until mid 50s. possibly at sunken temple you may run into problems, but you're right until then. a shadow priest can heal just fine.
quickly, as a shadow priest, i'd only do dps if
a) i'm not the main healer. that means shadowforming and topping the damage charts.
b) the party is quite high level and can handle the mobs easily. i will stay in healing form, but use shadow spells until about half mana is gone, then hold that in reserve for healing.
watch out for how you use vampiric embrace. if there is any question of the tank holding aggro, you shouldn't use it. i often found that i would only use it when there was only a single target - like in boss fights. otherwise all the adds come to you.