Hi,
I crossed in and out of Laos here earlier this year, via the new bridge. Coming into Laos there was a lengthy wait - not because there were many people buying visas, but because all the Laos border staff were eating their lunch, and everyone - regardless of whether or not they had a visa already - had to wait for lunch to end before we could get stamped into the country.
In a way, you're not over-thinking the problem. In Vientiane, the queues at the border to buy visas are sometimes very lengthy (1-2 hours), and getting a visa in advance would save you some time. But in Huai Xai, at least from my personal experience, this isn't the case. Getting your visa at the border should be fine. The main issue with this border crossing is the new bridge, which makes transport inconvenient. It's the best part of 10km or so from immigration at the bridge to central Huai Xai.