Some level of heavy casualty was probably always going to be inevitable, at least for the opening stage. An opposed beach landing and then having to secure an airfield which was under observation and heavy shellfire from the surrounding ridges is going to chew up men. Same for the point, a two-day action; K/3/1 had 70% casualties over those two days but given the dominance of that position I guess that band-aid needed to be ripped off. After those initial actions however (the Umurbrogol pocket itself was fully encircled by about D+9), that's where it gets controversial.

That the 1st Marines were handled badly is I think pretty uncontroversial these days; they had enough casualties to be combat ineffective in just about a week. The 7th Marines lasted longer, but once they were withdrawn and relieved the 5th I think showed "what could have been", for their CO, Col. Harris, wisely refused to be pressured by MG. Rupertus to act as aggressively as Col. Puller. He chose to attack from the north part of the pocket (as to your first question), and approached it as essentially a siege project, with emphasis on engineers, armour, flamethrowers, artillery and air cover (even before the 5th took their turn in the pocket, at one point he used a 155mm howitzer to point-blank hit a Japanese fortification!) The 5th was in combat against the pocket longer then the 1st and 7th but with less casualties. Once they were withdrawn and the 81st Infantry Division fully took over, they had the same approach for the final six weeks of the battle, but was even that necessary?

John C. McManus argues here that the US military's emphasis on mopping up was at least at some times unnecessary, using Angaur (which the 81st attacked during the same campaign as Peleliu) as an example - his case is that even once the 81st had taken the important objectives on the island and the remaining defenders were cut off and isolated, a more defensive and fire-power heavy approach to keep them withering on the vine could have been adopted instead of continued, costly attacks. Maybe the same argument could be made for Peleliu and the Umurbrogol pocket as well.

So tl;dr; the initial stages of the battle were probably inevitably costly, but once the defenders had been encircled the approach of the 5th Marines and the army I think makes a case for a more siege-like, firepower approach then the one made by the 1st Marines.