Jim,
I like spelt and when made with honey its lovely. Likewise I like the slightly earthy rye flavour and certainly the texture. I've replaced honey in the past with hazelnut syrup (like the stuff you get for coffee) and that was lovely
I think adding molasses would be worth a go but perhaps lessen the improver slightly also due to the extra bit of sugar. Stay with your revised amount of yeast though.
Ratio? Probably 75/25 is a good start due to flavours but if you getting short of the improver let me know (PM address), I go away on Friday for a week though