>> I did think of moving the item away from the inside gutter, and a quarter point is sufficient, but it’s technically wrong. If I print a spread it will have a hairline of the background color missing.

>>> But the pages fold (thus the spine). It won’t be seen.

This has been a problem in InDesign for ages… will they ever fix it? I’m one who likes to use precise measurements so having to reduce the width of a full bleed background by a slight increment gives me a nervous twitch!