The closest airport to Porthmadog with reasonable international connections is Manchester. You can take the train from Manchester to Shrewsbury (1hr 16min) and then change trains to Porthmadog (3hr 26min). There appear to be departures about every 2 hours.
Weather in the UK in July can range from hot and dry to cold and wet and predicting it in advance would be futile! There is a reason why England is so green and there are flowers everywhere!
Portmeirion is only a few miles from Porthmadog and although incredibly touristy is also a pretty cool place and if she likes to shop, she will be happy. Check it out on the web.
The observation car is on the tail end going up to Blaenau Ffestiniog and next to the loco returning.
Driving in the UK isn't hard if you have good spatial relations. Basically, everything is reversed. Right turns are like left turns here and vice versa. I find that leaving the airport in a rental car you are almost immediately on a dual carriageway motor way (divided highway expressway) so you can hang in the left lane (the slow lane) until you get comfortable. Exit at the various service centers along the way to take a break, but to also get used to going through roundabouts. The drive from Manchester to Porthmadog is on motorways for the first half, then is on two lane roads the rest of the way. Take it easy, watch your mirrors and be especially careful when turning onto a cross street to watch for traffic. Put your head on a swivel until you get the hang of things. I'm 71 and hadn't driven in the UK for 11 years and felt comfortable within 30 minutes of leaving the Manchester airport. Do bring a satellite navigation system, particularly one with speaking instructions so you can concentrate on the driving. Or hire a car with a nav system. It will tell you which lane to be in and which exit to take when you encounter one of the ubiquitous roundabouts. I find UK roundabouts to be much better signed than ones in the states.
If you are still nervous, take the train and arrange a car and driver while you are in Porthmadog.
Seating in 3rd class is comfortable with padded seats, but somewhat upright.
Michael Allen