I could be wrong, but the ones I’ve taken apart in the past have the brake side bearing in the main hub and the gear side bearing in the freehub. The 10mm bolt tightens the splined inner, fixed part of the freehub to the main hub and makes it all one solid unit.
The freehub has some separate small bearings in it that lets the outer shell rotate, which are non-serviceable as it’s a sealed unit you unbolt and throw away when it’s knackered, so I think that it’s likely that your has some kind of manufacturing fault from new that means it needs replacing. (I don’t know if you can picture how this all works from my description, not sure I could! 😀 )
If that 10mm bolt was loose, I reckon the whole freehub would be wobbly on the wheel as well as the side to side play you describe.
I’m like you that I’d rather just fix things rather than have the faff of taking it back to the shop, but in this case if you dismantle it all and the shop can tell you did this when/if you take it back they’ll just say you caused the fault and it’ll be hard to get them to honour the warrantee.