Effect of frequency division on a signal's spectrum?
You can't just divide the frequency of an arbitrary signal. Think about this pathological case:

Imagine you have an RF signal at 100MHz occupying a bandwidth of 1MHz. What happens if you frequency "divide" it down to 100kHz? You can't, because then you wouldn't have enough bandwidth at that center frequency.

You can however use frequency shifting, but that's basically what a quadrature modulator does. 

So if you tried to use frequency shifting, you'd need a quadrature modulator after the output of the LTC5598, so that's not exactly productive.

I suggest you get a different quadrature modulator, or (especially since your CF is low), use direct digital upconversion by just blasting the signal with a fast DAC. A 10MHz DAC can pump out 0-5MHz, so use something like that.