1 - Yes. With attention to detail by both you and the board house, you can move a hundred amps with +/-10% sharing.
2a - The internal traces will run warmer than the external ones. Some online calculators address this.
2b - Pay attention to the vias; they have a large impact on how evenly the current is spread among the layers. Depending on the board stackup, you might have 2 oz copper traces but only 1 oz copper in the vias. If you want to get picky about it, a via is a very short trace whose width is equal to the hole circumference. Use the hole diameter minus one plated copper thickness as the diameter. This is equivalent to the average of the inside and outside circumferences of the copper tube.
Also, while it is good to spread out some vias along the trace run, the clusters of vias around the ends (or any high-current branches) have the biggest impact on current sharing.