I think you should do it in 18kt white as it will blend better with your e-ring. The diamond band, while in platinum, stands on its own.
The 18K white gold will rub off the rhodium and it will look "yellow" pretty quick. At least with the plat you won't have that. This is speaking from personal experience.
Do you think this will happen with just a low simple band? I’ve had stacking bands and never had an issue. I’m thinking as long as the band falls below the prong line so the prongs are not grinding against the band... what do you think?
Try a rose gold spacer. It will be like "nude"...
They sell what I call a spacer at Tiffany’s, it is sterling silver, my wife uses it. I have attached picture.
Here is a Tiffany link to something close, they don’t sell the exact one anymore
Rings for Women | Tiffany & Co. US
Tiffany rings are renowned for their exceptional craftsmanship and timeless style. Explore classic signet rings or create your own ring stack.www.tiffany.com
Yes, my wife’s space from Tiff is Sterling silver. I notice they are out of stock now, ugh.
Also check out eweddingbands.com, they have a lot of variety for 1 or 2 mm bands in different metals. Good luck.
Also checkout eBay for inexpensive or you want to try a size before you spend serious money