The symptoms Bob suffered before he died are identical to aconite poisoning, which is why it is a major line of inquiry now. It would also explain how such a physically imposing man, at 6ft 1in tall, died without putting up a fight. You'd struggle to get two people into his bathroom let alone three, so it could be no one was there," he added.
Please explain the bold highlighted parts
major line of inquiry - primary line of investigation
You'd struggle to get two people into his bathroom, let alone three, so it could be no one was there," he added. The bathroom was so small it would only hold one person comfortably, two with some difficulty, and three not at all, so possibly there wasn't anyone else there.