Hello there!
1: The max width is 120, but very rarely do maps hit the maximum width. You will be fine at 105!
2: You can get individual piston parts by placing the piston how you'd like, then activating it. then, set your 1st and 2nd worledit positions on the part you want to erase, and replace with air.
3: You can't use the code "water" on ManaCube, but you CAN use its numeral code, which is 8. you can //set 8, or what I usually do is set wool how I want the water for jumps, then //replace wool 8.
Hope this covered it!