This should be my final update in this thread. Both problems have been solved.
For the steering, I followed @Timr71's suggestions and contacted Ron to get a new steering cable. Because there was also a bit of vertical play in the tilt steering assembly, I ended up replacing that, too. The new setup is solid as a rock, and it steers on a dime.
For the loss of throttle, I followed @InfinitySurf's suggestions and followed the fuel. The carb looks okay. The water separating fuel filter was good. The culprit was the fuel pump. After putting the new fuel pump on and replacing the fuel filter with a new one, we took the boat out for a test.
We were slowly driving along for a good bit at a slow speed. We turned back towards the ramp (so we'd at least be facing the right direction) and gave it some throttle. The boat responded and didn't let up. It was great! I kept waiting for the engine to die out on me, but it never did... it ran beautifully!
MANY thanks to both Timr71 and InfinitySurf for your help! (InfinitySurf, checking the impeller is still on my list).