The point of having editable initiative is ease of using delay or ready action. One certainly can use arrows to move combatant up or down in the turn sequence, but its initiative wont change. For example character with initiative 10 will act before character with 18 one. It's counterintuitive and in complex combats with many participants can easily lead to DM mistakes.
In my opinion it is much more easy to edit initiative directly and then sort combatants list accordingly. When DMing I usually use Excel Visual Basic script that sorts combatants list after I have set or edited initiative.
Using Excel as DM tool is very convenient because I can store a lot of different info such as session notes, remainders, log of crucial events happened with my players in campaign, etc, and at the same time I can add buttons with VB scripts and other simple programmable controls, all in one application. But that requires some basic programming skills, so not everyone can use it.
That's why I recommend you to add sorting of combatants list when focus leaves initiative edit box or when Enter is pressed if initiative for selected combatant has changed. Also sometimes you can have a situation where a delayed combatant wants to act between two other combatants with consequent initiatives - 17 and 18 for example. To resolve such case I recommend using fractional initiative instead of integer.
And I found a couple of bugs:
1) The program crashes with 'invalidoperationexception'. When monster description is shown there are lines that separate defense and offense sections. When trying to select that line with text selection moving the cursor from its right edge or from some place to right edge repeatable crash is encountered. It happens only if no text is added to selection. Try to select different parts of that separators to encounter the crash.
2) When many players are added to party list the scrollbar appears. The scrollbar cuts some part of player boxes from the right. Also I have experienced lags when using this scroll by dragging the mouse. Same with monsters list.
Thanks for improving this program!