Lots of good questions. First, yes, the Promax will accommodate two anodes. I’d never thought of staggering the anodes, but you could do that.
As to needing a curved dip tube, if you’re in a hard-water area, I think you should get one because we don’t think self-flushing dip tubes work very well. They try to make sediment float out with the hot water, but sediment is heavier than water.
Combo anodes do restrict water flow somewhat. The outlets are 1/2-inch by 3/8-inch, although there are two of them on either side of the nipple assembly. If it’s an issue, you could buy one extra-thick anode and simply check it every few years. That’s what I do with mine.