I second everything here! The Mordred/Merlin dynamic was really interesting to me and what held my attention the most, since my OTP had so little screen time this week. I don't really see the sexual tension between them, aside from them both being extremely attractive, though there is of course tension and chemistry because the actors are so amazing. And I think that's because while enemies often have a very explosive sexual undercurrent, Merlin's antagonism towards Mordred is solely based on his love and fear for Arthur's safety. His reasons for keeping Mordred's secret seem quite different from Mordred's, since Mordred could actually use his knowledge of Merlin's power to discredit him in A's eyes, and I Merlin's motivation has to be based to a great extent on self-perservation. In any case, it will be interesting to see how it plays out, because yes, as you've stated above, Merlin's obsession w/keeping Arthur safe is showing itself to be his main vulnerability. He's making decisions that are OOC based on it, and so something will have to give at some point.

Let the alien die!