Conditional grey-out and disabling of cells

IT'S SO CLOSE! It works really great, just a couple more small things, and if I knew anything about programming I'd tweak them myself, but i don't!

1. When you select anything other than "DI," "LTC," or "Z" it unlocks the whole sheet and then all cells on the worksheet are un-protected. Then when it re-protects, it forgets which cells were locked and which weren't, and leaves all cells unlcoked. Is there any way to get it to immediately lock again when one of those 3 values is selected? The sheet needs to stay locked at all times.

2. And the other things is, let's say when you select a value in E17 such as "DI" and F17 get's greyed out and locked, then you move down and select "DI" for E18 and F18 gets locked, F17 re-opens. I need for all the greyed out cells to stay locked once their locked. It only locks the last cell that you work with. And i don't think I can manually lock column F because if the value IS NOT one of the 3 chosen, I need to be able to enter information still, and can't do that if the sheet is locked like it will be AND all the cells are locked. So I need all greyed out cells to stay locked, all non-greyed out cells to stay unlocked, and the sheet to always remain locked and password protected.

And a couple other wierd quirks.

3. Whenever I highlight at least one E cell and one F cell (even if they are blank) and hit "delete" it gives me an error message, "run-time error '13.'"
4. Also, when I manually type in the values instead of using the drop down list, the wrong cell gets affected (ex. if I typed "DI" in E17, F18 would turn grey and lock. And vice versa with typing in a value other than the 3 chosen, the wrong cell turns white.

5. And when I clear out a cell and it's blank, the corresponding cell stays gray. So maybe that goes along with the last error and when you type stuff in it gets messed up.

6. And one other thing (not too important) is when I hit Ctrl^C to copy a cell in column E to paste in multiple other cells, it only copies once, making you hit Ctrl^C again instead of doing multiple pastes. This is last b/c it's not super important. If these few quirks get worked out it will be PERFECT! Hopefully it's not too much tweaking. It just needs to look very professional while at the same time being user friendly for the people inputting info. Thanks so much for all your help!