is there any way of estimating how full this train will be (Tuesday morning, July 17)?
One can estimate based on experience, but one cannot predict.
It makes about 11 stops between Llandudno Jct. and Wolverhampton, so if we opt for unassigned Advance seats would we be moving around to avoid seats that are reserved between intermediate stops?
This train is the 0923 Holyhead - Birmingham International 1350, which makes up to 25 intermediate stops, 8 of which are request stops which are made only when required. The full itinerary is
here.
It appears you can't view a diagram showing reserved seats on ATW like you can on CrossCountry.
That is correct. Sorry.
A dummy reservation for one person yielded a seat assignment of 01B, but does that indicate anything this far in advance about how full the train will be on July 17?
It suggests to me that very few seats have so far been reserved, but tells me nothing about the number of "places" which have been reserved without a seat being allocated and does not give any useful indication
about how full the train will be on July 17.
My expectation would be that the 0923 Holyhead - Birmingham International on Tuesday July 17 would not normally be very busy, but I would not expect to find a free table of 4 seats if joining at Llandudno Jn at 1025. But even the evening before travel, it will not be possible to predict with certainty whether your group will have a comfortable journey. A sunny day, the cancellation of a previous train, the closure of a road or a delay to a ferry from Ireland could all add large numbers of passengers to a 2-coach Class 158 diesel multiple unit (DMU).
Since ATW replaced seat reservations with place reservations for Advance tickets sold online in December 2009, I have made half a dozen long distance return journeys to Wales involving several hours on ATW DMUs each time. The party size has ranged from 2 to 8 (6 adults and 2 aged under 5). On one occasion we travelled at short notice and had flexible tickets including seat reservations. On all the other occasions, we had Advance tickets with place reservations only.
We have always obtained seats with the single exception of a connecting train from Llandudno Jn to Bangor, so I have had to stand for only 20 minutes in journeys lasting over 20 hours in total. It has not always been possible to obtain seats together.
My experience is that holding an ATW seat reservation does not completely eliminate the risk of having to stand. When we travelled with flexible tickets, the reservations were not placed on the seats for one of the two legs of the journey. I could say that this is a failure rate of 50%. Hopefully, your experience will be different.
If I were a person for whom holding a seat reservation was a very important part of planning a long-distance railway journey, I would buy Advance tickets for the 1025 Llandudno Jn - Wolverhampton on 17 July, together with seat reservations at a railway station, not online. If, exceptionally, that railway station was unable to provide seat reservations with Advance tickets for that train, I would not complete the purchase but would try another station.