Rather than derive the 6,5x54MS from Krnka’s 8x45 ( same head diameter), I would follow the 6,5x52 Italian Rimmed trials
cartridge of 1889-91. Part of the initial trials tender was for a rifle chambered for a Rimmed cartridge compared to the same model chambered for a Rimless cartridge., The Calibre was initially set at 6-6,5mm, but by the time the tender closed, only 6,5mm examples were submitted.
Mauser of Oberndorf submitted two examples, of a design similar to the 1890 Turkish Model ( five shot inline magazine), for both rimmed and rimless cartridges…other submissions by the Italian Royal Arsenals and other makers were also tested.
After the first Tender, the specifications narrowed down to the Rimless cartridge, a Mauser-type solid Bolt ( all the two piece bolts were eliminated) with a Mannlicher-style symmetrical clip similar to the German Commission 88 clip, but because of the smaller case diameter, holding 6 cartridges instead of 5 ( as in the M88, or other larger Mannlicher clips). The Rimmed 6,5x52,5 case just disappeared from the horizon in late 1890, only to appear a year later in the Romanian (Steyr)M1892 Mannlicher rifle, cal 6,5x53,x,R…except for the rim and case length tolerance in manufacture, identical to the Italian Rimless cartridge. This cartridge was also then used in the definitive M1893 Romanian, and the Improved M1895 Netherlands Mannlicher.
In 1897, Portugal was casting about for a new Steyr rifle design, and cartridge (its BP 8mm Kropatschek was definitely "passe’ ", and adopted a small batch of M92/93 Romanian rifles and carbines for an extensive trial ( Portuguese M1897 Cartridge)…after the army trials, these were relegated to the Police (Gendarmerie) and Portugal also tried a batch of 400(?) M1900 Mannlicher Schoenauers in 6,5x53,6 Rimless (AKA 6,5x54 MS ). These were returned to Steyr in about 1903, and in 1904, Portugal adopted the Home-designed, DWM-Engineered Vergueiro (effectively an “Improved” M88 rifle)
with a high power 6,5 cartridge ( 6,5x58 Portuguese §) and a Mauser staggered internal magazine, and M1904 Commercial Mauser sights.
The Greeks, Long term buyers from Steyr, adopted the M1903 MS, and some of the ex-Portuguese M1900s ended up being sold to Greece.
As to the Sporting Connection,
British, German and Austrian Gunsmiths had been buying barreled Actions of the M92/93 Model from Steyr, and “setting up” light sporting rifles. especially for Mountain Deer and Chamois hunters, and even as Target rifles for Bisley, using the original 6,5x53R Romanian/Dutch cartridge ( “.256 Mannlicher Rimmed” (or “Flanged”).
It was a simple transition by Schoenauer, for his Rotary magazine concept, to use a rimless cartridge for ease of feeding,( he had designed a rotary back in the BP days, for rimmed 11mm BP cartridges)
THus the rimless 6,5x54 came about as a simple solution for Non-clip ( ie Mauser) magazine type rifles, and was the cartridge of choice for the Mannlicher Schoenauer (Both Sporting and Military.)
Does Von Kromar shed any light on the development of the Shoenauer rifles??? ( I know his book was dated 1900, but any 1898=99 info should have got in, if it was of sufficient importance.)