no, I think a really big and stronger cyberdemon would be much better, both of them are really similar. I think replacing Mordekai with the icon of sin would be the better choice

Time will tell

I totally think Icon of Sin will replace Ugh Zan because his Eternal counterpart is not a wall but is an armor suit with a HUGE demon in it that looks true to its design. So, unless you adapt the other worlds besides egypt, you can do that. Or you can use a titan which is similary big for Ugh Zan and save Icon for Mideval when your doing second encounter. Whichever you feel is best

maybe during the the first ugh zan encounter when he chases you to the pyramid there should instead be a bunch of demons, then you meet the icon of sin inside the arena at the top.