Question: "I am wondering why the firing pin of this "M97 action" looks so much shorter,
when the standard-length M98 bolt is interchangeable?
Is this a photographic illusion?"

Mostly photographic illusion, as I took the photos at an angle to get the essentials. In part for another reason: The firing pins are of the same length, but the 97 one has a longer firing pin travel than the 98. As both assemblies are shown cocked, the 97 one is farther back relative to the bolt sleeve. As the rear end protudes more, the front end seems to be shorter.

Question:" was the pre transitional action also sold to england? I believe we had sporting rifles with transitional actions by british gunmaker before."
I don't know. Apparently nobody has looked close enough at the very early .275 Rigbys and others. This one seems to be a .275 Rigby on such a "97" pre-transitional cock on closing action, going by the checkered safety:
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=218762&an=0&page=8#Post218762

. The 1896 tests tried two action types: The cock on closing "model 97" shown here and the advanced, cock on opening "98 transitional" type. "True" transitional type actions or transitional bolts appeared here before, both in intermediate length as used by Rigby or standard length.
See these threads: both show early Mauser 98 rifles with transitional bolt bodies, firing pins and firing pin nuts with later bolt sleeves with sleeve locks, in rifles with 98 large ring receivers.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=231141&an=0&page=1#Post231141
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=231334&an=0&page=2#Post231334
As a transitional bolt body does not have the recesses for the safety firing pin lugs on the striker 's collar, and Mauser changed the interrupted threads at the rear end slightly, you have to use a transitional firing pin and nut in a transitional bolt body. As you see, these three parts can be combined with a "modern" bolt sleeve with lock.
Cock on opening transitional 98 actions were shown here:
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=247307&an=0&page=0#Post247307
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=90810&an=0&page=10#Post90810
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=91206&an=0&page=10#Post91206
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=106278&an=0&page=26#Post106278

Question: "Is there any way that a cocking peice from a '96 can be used on a '98 bolt?"
No, see the photos. Additionally, Mauser slightly changed the interrupted thread from the transitional to the fully developed 98.