As a healer, your first priority is to keep yourself up, first and foremost. If you go down, your team goes down. Of course, the situation could force you to prioritize a little differently. For instance, if I have a low health ball carrier that's about to score, I might prioritize them over myself to help them make those last few steps to the goal line. I saw a great example of that last night.....The ball carrier made it across the line and went splat just as he crossed. And then I went splat. It was awesome to see.....And it earned us the win. But generally speaking, you'll want to prioritize yourself first.
Umm no, your priority is to keep the GROUP alive, not just yourself. This means that you always should prioritize whoever has the lowest health first. The tank guarding you is likely getting hit by their ranged dps and is also taking the damage that the melee dps is doing to you while trying to interrupt your heals etc. If the person guarding you goes down you go down. The situation awareness is much more in depth then what you have described.
One of the things I love about the vanguard is it is easy for me to guard the healers and infact it should be the ranged tanks job to guard the heals. It is much more important for a melee tank to rotate their guard since it is harder for them to stay within the 15m range of whomever they are guarding etc.