I believe you are referring to Baby A here.
First of all, I have not read that Baby A was 29 weeks. Do you have a source for this? The nearest I could find was the Guardian article describing the twins as 6 weeks early, which usually means 34 weeks. There is a big difference between 29 and 34 weeks. I'm working from the assumption that the twins A and B were 34 weeks. That gestational age could stand 4 hours of interrption in IV fluids. But even at 29 weeks, while that is quite a long time off fluids, I would not expect dehydration to the point of circulatory collapse. Even if he was dehydrated, the treatment is starting IV fluids. If dehydration caused the collapse, giving fluids would have helped.
Regarding the blood clot hypothesis-
From the trial, the peripheral cannula tissued at just after 4pm. Two attempts were made to place a UVC and in both cases the UVC was removed because they could not obtain ideal placement. Then the long line was placed and fluids commenced at just after 8 pm.
Getting the UVL to go where you want is equal measures luck and skill. After two attempts they gave up and sited a long line. This is reasonable.
In the meantime, nasogastric milk feeds were commenced, which was appropriate. They were small volumes paving the way for larger volumes. Keep in mind that the volume of IV fluids per hour would also have been small.
This timeline suggests that no line was in place for much more than an hour. That's not very long. The lines would all have been saline locked and flushed intermittently while placing them and while waiting for xray.
The fact that they were able to start fluids suggests the long line was not clotted, because clotted long lines are usually occluded.
I don't think the clinical details relayed in testimony line up with Dr. Lee's central line related thrombus hypothesis.
Something can be not ideal or not a best practice while also not being a plausible cause of death.
THE prosecution in the trial of Lucy Letby, who denies murdering seven babies and attempting to murder 10 more at the Countess of Chester Hospital…
www.chesterstandard.co.uk
How events unfolded in neonatal unit where killer nurse worked at Countess of Chester hospital
www.theguardian.com