Fresh information in the quest for knowledge. After 30+ min. on the phone with the HF people we have determined that the Hub and Bearing for the two trailers- item #42709- 990lb trailer and 90154 the 1175lb trailer- the parts are the same- identical part# for the bearings and hubs (for those inclined toward humor let me be more specific- the bearing and hub each have their very own part number but the two trailers share the same replacement parts- okay now I’m just confused- you know what I mean- leave me alone.)
Therefore I (or you dear reader) can clearly use the 12” wheel on the 8” trailer –

But as we have been warned- adjust the fender accordingly.

So… what about the leaf springs? Glad you asked
They don’t know- yet. I strongly suspect that the springs are interchangeable- if not identical.

This might lead to question #3- If the springs are different, is it worth the cost ~$10 each to swap them out and gain around 200lbs capacity?
Your humble servant- Roger
Those in the know might choose to include this information in the appropriate trailer info place- unless I just missed it and I am duplicating work