Final Result:
Seat belt extension plus new buckle (shorter to allow for better fit) done at Stanmore Quick Fit. I opted for the installation services at GBP120. You can also order for self install.
Final seat selection is Maxi Cosi Pebble.
Passenger seat is usable for my wife and myself (I am 174cm tall/short).
The Process:
With the seat belt extension in place the Hauck Carseat 0+ fits (Not to be confused with the Hauck Zero Plus, which is a different model).
Being unsatisfied with the build quality of the Hauck I mapped all currently available infant seats according to their dimensions. Critically I focused on those with available test result, especially crash tests.
The important variable is the length. The Hauck unsurprisingly is one of the shortest out there with a length of 63cm.
The other candidates are as follows:
Recaro Young Profi Plus - 62cm (upright) - 73cm (reclined) - Decent fit and definitely the smallest in the field when in the most upright position. When in the upright position it is relatively steep though. There also is very little space between seat belt and where the baby is meant to go. For those seeking maximum passenger seat space and are fortunate enough to have a small baby this might be an option. Sourced from Halfords.
Kiddy Relax Pro - 63cm (upright) - 73cm (reclined) - I didn't get to test this seat. I assume the fit would be similar to the Recaro. I had ordered one via baby-pages.co.uk though before opting for the Pebble and cancelling the order.
Cybex Aton - 63cm - Virtually identical to Hauck 0+ in length but much wider and with much better quality. Good fit. One of the best reviewed seats on the German market. Sourced via Halfords. Downside here was the relative steepness of the baby's position and again the small space between seat belt and where the baby will be. Otherwise strong contender.
Concord Intense - 64cm - Halfords has them on their website but they didn't have them in stock.
Britax Baby Safe Plus SHR II - 65cm - While it fits, it is already too big given equally or better reviewed but shorter seats are available. Sourced from Halfords.
Maxi Cosi Pebble - 69cm - I only included the Maxi Cosi in the group originally due to its compatibility with the Bugaboo Bee Plus (which is the pram we will buy as it fits in the trunk of the 993) and its excellent test results. At a length of 69cm I didn't expect it to fit. As almost all other seat options are at or longer than 69cm it is a natural cut off though.
Surprisingly the seat did fit at the same passenger seat position as the Hauck 0+ and other comparable seats. The reason is the Pebble's much flatter seat position and overall lower profile. This flat profile makes the end of the baby seat's headrest touch the back of the 993 seat below the bulge at the headrest of the passenger seat. The extra 6cm therefore easily sink into the seat. This is further helped by the natural inclination of the passenger seat. I might have to worry that the leatherette on the back seat will stretch a bit but I decided this was a risk worth taking.
So with the Pebble you get a seat that offers a stable fit, a preferable flat seat position for the baby, and enough space in the passenger seat for either my wife or myself (at 174cm) to sit well enough for even extended periods (keeping in mind that you shouldn't keep the baby in a car seat for longer than 2 hours anyhow). Sourced at Halfords but available almost everywhere.