Surely, the ‘problem’ with Pembrokeshire is that the rail routes split into three = Pembroke Dock/ Milford Haven & Fishguard. Unfortunately, the county town of Haverfordwest is only on the Milford branch. So, what would serve the populous best as well as tourists? The key is surely Carmarthen as it is west of this town that the 3 Pembrokeshire routes diverge with the first junction being at Whitland. Surely, most people on each of the Pembrokeshire lines would like to have one train per hour - which would mean relatively short trains (sprinters or 2 car 170’s or 175’s) west of Carmarthen. Then they would like a very fast train to get them to/from the east without going into Swansea. I would suggest that such a fast service could be a direct service to/from London using a 5 car Class 800 to join with another unit at Cardiff or a fast train heading to/from Manchester. Such a train could either use the Landore link line or the Swansea District Line - and only stop at Llanelli, Morriston Parkway (M4 J45), Port Talbot, & Cardiff before heading off to Manchester/ London.
Some of the sprinter services from Pembrokeshire could continue to/from Swansea as all stops trains, hence, Swansea retains its direct links with Pembrokeshire whilst with careful scheduling, Carmarthen becomes a major interchange point allowing passengers to switch to from the very fast services to Cardiff and the east. (At the moment, it is virtually impossible for tourists to make day trips from Cardiff to Tenby due to the excessive travelling time by rail. Ideally, an express train would leave Cardiff at 9am to reach Carmarthen in about 1 hour - where passengers could change to sprinters for Pembrokeshire - with possible train splits at Whitland & Clunderwen and the sprinters so configured to match loadings).
I know this post is about the Narberth Tunnel - but what I say is relevant because of the issue of taking Class 800’s through the tunnel and down a very slow branch line. (About time it had new long welded rail. Looks to me like the track has been around since the days of steam).