I am new, but read everyone's comments with interest as it's such a weird case. I am a single woman, and have been surprised (should be flattered but no I'm not), at builders etc, who have decided that because I'm single, I would want their attentions. They've been very mistaken and it's been very embarrassing for me and for them. Imagine how that would be for someone like MS, who is a friend and married, and what we don't know is, he may be narcissistic and obsessive and not take rejection.
So Here is my theory:
That after Samantha's relationship breakup with John Peake, Michael Stirling became infatuated/obsessed with the thought of Samantha living as a single woman alone. And perhaps even fantasising that she is now single because she has an interest in him.
Taking the theory further, Samantha would inadvertently have given him reason to think she felt something for him by getting him around to build the summer house, which was simply a genuine job she innocently asked someone she knew to help her with. Stirling then sees her everyday and continues to build a fantasy in his head that she has feelings for him. At some point, he tries to make a pass and declare his interest/passion. (This has happened to me with builders. Very embarrassing and awkward forever after.) She will have been surprised and rejected him outright - he's her ex s-i-l's husband. And no doubt, she also has issues with the summerhouse project as well and rejects that also.
Together, she may have rejected Stirling as a man and rejected his work, and probably not wanted to pay him either. He may be worried she'll tell his wife. These combined to make him threaten her - saying she led him on etc. Getting him around to build the summerhouse, being friendly etc. His rejection has festered into hatred. Resulting in him going to her house to await her return from work and confront her, demand payment maybe, at which point he's also attacked her in a rage and/or abducted her after first injuring her.
My 2p worth.
An excellent analysis and I concur with everything you say.