With regards to whether NB might have made it over the weir, and therefore
potentially out to sea, I wanted to have a look at what the tide was doing at the relevant time. According to the
tide times, the tide at Fleetwood (which is the end of the estuary where the Wyre meets the sea) was at its lowest almost
exactly at the time NB is feared to have entered the water up at St Michaels.
Had she crossed the weir, the water on the other side will therefore have been at it's lowest (which does reduce, but not eliminate) the chance that NB will have been able to have had a straight/fast route out to sea because that (tidal) stretch of the river does get very low at low tide with plenty of mud flats etc.
More importantly I think though is that if the tide was at its lowest at Fleetwood at around 9.30am on the Friday 27th, this means that it will have been coming IN for most of the duration of the day (up until its high again at 3.30pm). In other words, well into the search operation, the tide was coming
in at all points beyond the weir.
I live by the Thames in London (admittedly a much bigger river), but when the tide is coming in, tidal rivers effectively flow in the opposite direction - sometimes the effect is so large you can even see it on bouys or anchored boats. All this to say, if NB was past the weir, IMO, it is exceptionally unlikely that she would have gone very far, at least until the turn of the tide again at 3.30pm that evening.
Edited to add that this is all in my opinion.