Might be worth extending the case hardening beyond the end of the spline so that the edge of it doesn't coincide with the section change. You may also see benefit from using a tougher grade which can be used at higher strength whilst maintaining the toughness you get from 4340 at your current...
The test is pretty straightforward, a number of commercial test labs could do it on a sample or 'on site'
Try mailto:cts@allvac.co.uk if you're in the UK
The grade Ti-38644 has been used to make high strength springs. Data is available from RMI or Allvac, you should be able to find them from Google.
(ps make sure you isolate the graphite to avoid galvanic problems)
JSMAT
Mb can be used in a similar way to V to improve structure, but at 0.02% it would have little effect. The variations could be explained by variable scrap sources or carryover from previous heats.
You mention 'condensate', what temperature do you expect?
3.5% Ni alloy steels are not 'stainless' and are not renouned for resisting corrosion in hot humid conditions. The main effect of the Nickel in this alloy is to increase toughness rather than corrosion resistance.
