Blood only gets watered down if mixed with water when its fresh. Are you saying Sheila stood over the bucket and just waited for the lining of her uterus to fall in? 
I'd hazard a guess, that changing my mind from innocent to guilty, has in no way changed the fact that I probably have way more experience, than yourself, of soaking bloodied items, however, you post HAS made me ponder, again, on how long ALL the items had been in soak. I think it's safe to assume that the boys' clothes -their trousers- were placed in soak when they took them of prior to bed/bath time? Perfectly possible that the undies could have been put into soak at the same time. Aha! But there were TWO pairs, I hear you cry. Yes, you're right, there were. Absolutely no reason why, because Sheila was going to be away from the house that afternoon, that she hadn't worn two pairs to afford extra protection or "just in case". Every little helps, you know!
We're told very little about the soaking water -but this is where my long experience of soaking comes to the fore- I never soaked in JUST cold water. I ran the item under running water, to wash away residue, before placing it in cold water to which I'd added detergent. This had the added effect of lifting the stain and breaking down the meniscus. We've been told all manner of stories about how heavy had been her menstrual flow, both you and Mike alluding to something heavy enough to constitute a miscarriage -even though it was confirmed at autopsy that she'd been in the early stages of menstruation- HAD this been the case, I imagine that ALL her clothes, from the waist down, would have needed to be be soaked. She'd been wearing, what were once called, Capri pants, which are close fitting. There doesn't seen to have been any such item in soak. Only the boys' trousers in a separate pail.
I have NO idea if detergent had been added to the water, but as supporters are always very keen to say that just because something isn't said to the contrary, it doesn't mean it isn't so OR didn't happen, I'm going to assume it COULD have been. Going along those lines, I'd defy ANYONE to extricate blood, from either pail or knickers, to place in a silencer to frame Jeremy.
