Going to one chainring: realign cassette farther in by redishing wheel?
You definitely don't want to redish the rear wheel. The dishing is to center the wheel between the locknuts. It is necessary to accommodate the cassette, but it is more of a function of the hub than the cassette or the cassette position.

If I'm reading the right you'd like to convert to a single chainring in front and adjust the chain line (to center the single chainring on the cassette) so that you can utilize all of the cogs on the cassette while maintaining reasonable chain deflection.

The usual way to do this is by adjusting the length of the bottom bracket axle, but since you'd like to minimize spending on the project you're wondering if you can do this with the cassette. As best I know the only way you'll be able to do this is to drop on cog from the cassette and replace it with a spacer, so that you're moving the cassette relative to the chainring.

I think you could do that, but you may want to start by just using one chainring and see if you're happy with the bike in that configuration. You may be running with more chain deflation than would be ideal, but it should work. Then if you like it consider your options for improving the chain line.