1. That's only true in a recession. In a full employment economy, the macroeconomic effects will be more or less the same. If there's a difference, it will likely favor tax cuts on the rich (depending on your definition of rich) because the rich are typically still working. So the disincentive to work applies in one case and not the other. Now, I happen to think that the "marginal taxes on income disincentivize work" effect is small, which is one of many reasons I'm not a conservative, but small is better than zero. Note: the tax cut, unlike the Trump tax cuts, would have to be on work and not passive income, but anyway . . .
So the case here for SS tax cuts wouldn't be economic but rather fairness. The old folks deserve to keep more of their money, more than the rich. Something like that.
2. There are plenty of folks here on SS who therefore know more than me, but I wouldn't imagine that the average SS recipient is paying much tax on SS. The SS payments just aren't going to be sufficient to kick the recipient much above the zero-tax levels. The SS tax cut would be more beneficial to people who have a bunch of investment income from stuffed IRAs, etc. So it would still be a tax cut for the well-to-do, if not the wealthy.