Answer to Question 1.
Research At the Oxford Orthopaedic Engineering Centre published in Spine in 1983 (1) selected a group without back pain or history of back pain to evaluate the mobility of the normal spine. It was by examining these x-rays of the standing and flexed positions that the limitation on the flexion of the lowest two discs became obvious.
At the University of Babol in Iran about 100 individuals were also chosen who were free of low back pain. The standing shape, including the angle of wedging of all the lumbar discs, was evaluated by x-ray and published in 2003 (2). The mobility was not evaluated in the same research but applying the mobilities from various publications, such as Adams and Hutton in Spine 1984 (3) or (1) above, the same limitation is found for the lowest two discs, and only for the lowest two discs.
A similar evaluation of the mobility of the normal spine was done by Jonk and Van Niekerk in South Africa (4). This was particularly valuable because the subjects chosen were black people who were not using cars and although they were workers at the hospital their lifestyle was probably far less "western" than any other subjects. Sadly, the x-rays had already been destroyed by the time that I asked to look at them in the late 1980s but Professor Van Niekerk obtained for me some new x-rays of tribal living Africans. These confirmed the wedge shape of the lowest two discs in the standing position. Combining these shapes with the published mobilities confirmed the limitation on the flexion of L4-5 and L5-S. The angles were similar to those of the British group though the wedging of the lowest two discs was slightly greater. (I see this as probably a lifestyle difference, not a racial one)
(1) Pearcy MJ, Portek I, Shepherd J Three dimensional X-ray analysis of normal movement in the lumbar spine. Spine 9: 294-297, 1984
(2) N. Kamali EVALUATION OF TOTAL AND SEGMENTAL LUMBAR LORDOSIS USING RADIOGRAPHIC.... Babol University of Medical Sciences. Quarterly Journal Summer 2003 , Volum 5 , Number 3
(3) Adams MA, Hutton WC The effect of posture on the lumbar spine. J Bone Joint Surg (Br) 67B: 625-629, 1985
(4) Jonck LM, van Niekerk JM A roentgenological study of the motion of the lumbar spine of the Bantu. South African J Lab Clin Med 2: 67-71, 1961