Hi
You've a job and a half there esp with the handrail and spindles,sometimes that waterbased stain is worse to remove than a solvent one.
My advice would be try a product called prepdeck i think if i remember cuprinol/ronseal do it,give it say 30mins to soften the stain and power wash off DONT get the nozzle to close your'll damage the decking and try keep an even distance or you risk striping,do this on a test area 1st round the back or summit.
See how you get on with the clean,then decide which colour and type would suit her best i'd go for a non waterbase though,theres loads out there to many to list.