Follow up: In the end I bought a Shimano 8-spd 11-32. I talked myself out of the 11-34 Megarange because in the 8-spd version there is a huge jump between the last 2 cogs, 26T to 34T. My wife tends to shift late, and it will be under load of pulling a kid trailer, so I thought the smaller step would be more important than the extra 2 teeth.
As somebody pointed out, 12T would have made more sense than 11T on the small cog, but the 8-spd does not come in that flavor.
I went with the low end Altus MTB derailluer to replace the Sora that came with the bike. I was looking for a transition, not an upgrade.
I had considered buying a cassette tool and even a repair stand, but in the end I chickened out and had the LBS do the work. The mechanic said he removed 2 links from the new chain (but he didn't mention length of new chain).
As you all predicted, the 8-spd MTB derailluer shifts perfectly with the Sora 8-spd brifters. I am thrilled with how it turned out. The wife has yet to ride it under load. But when she does I am sure to see some love.