No icons aren't easily replaceable, that's the point. There's no one in the narrative that could slot in & fill that void that the legend of Shep occupies.
Highly, highly subjective viewpoint. But no, the narrative didn't let that happen, even though the failure and death of icons inspire equally powerful icons to emerge in real life, as well as sparks to resolve conflicts and cooperate.
TIM goals are why Shep is back but that doesn't tie him/hero to do whatever whim he wants. It's not borrowed time, its a second chance just as someone who has a disease cured that was going to kill them gets.
Ah, yes. Being dead for two years is now tantamount to a curable disease. Forgot about that with ME2.
I disagree. Death + motivation for resurrection = borrowed time. Shepard's brought back for a reason, and it's not to make babies or go fishing while basking in their new health. There may not be an obligation to TIM, per se, but there's an obligation to his reason for doing so.
There's no suggestion in the game that Shep doesn't have to eat.
Why would Shepard have to eat with all that hardware? Who cares about metabolism now? Does Shepard even have a traditional stomach?
As to reproduction, given the ways we've seen in terms of advanced technology making reproduction possible.(Miranda's father) don't see any reason to assume it's made his genetic material unable to be used for reproduction.
Nothing at all indicating that this biosynthetic fusion and recreated tissue restored Shep's reproductive status to viable, either. Remember: meat and tubes, two years dead, significant synthetic work required. This would have to be conscious priority. Other options will always be available, though, so long as they have remnants of Shep's old DNA.
Now there may be issues regarding long term issues with strain on his re-animated tissue i suppose(given renegade tissue breakdown) but not the actual tech itself, we can have both the head of the project and the tech itself. So there's every chance imo of being able to manage it to give Shep a decent lifespan.
Assuming Miranda's alive, of course, and that Shepard continues to trust her.
And that's all best-case scenario, without any communication or connectivity issues involving this ghetto-rigged neural network.