Let me first say that there are many out there with much more knowledge than I, but I will attempt to answer some of your questions and hope that if I get something wrong that others will chime in.
1. I would not use more fluids than recommended for the Dales. I know that overfill can be a problem especially for the engine oil. I would just use the amounts called for. No reason to overfill and risk damage.
2. I have read about the 5th gear issue. Frankly, I don't have an issue with my 5th gear which is stock. With a larger front sprocket you now have "taller" gearing which will probably make the 5th gear issue worse. If I understand it correctly, the complaints with 5th gear is that it is too tall and makes for a fairly noticeable gap between 4th and 5th. Changing the front sprocket wouldn't make the gap larger, but it will make all of your gears taller. I would just try it out and see for yourself. You might try the stock front sprocket also and see what you think.
3. The D&M is computer software that allows you to make changes to the ecm. I also allows you to do diagnostic analysis of the quad for potential issues. You can also change maps, do throttle calibration, check RPM, etc. As you probably know, the Dale has EFI.
4. I don't know the contents of engine ice, but Evans is very highly recommended with the mag head covers. If you want to run engine ice, I would get the aluminum cover to be safe.
5. The oil hose mod is more than just moving the oil lines. They are re-routed to a different configuration. Also, a banjo bolt is replaced with an oil drain plug. Look at the Dale sites. There are very good instructions on the oil mod in the updates.
I hope this gets you started. Good luck.
'02 Cannondale Moto 450 w/updates
2010 RZR-S
'02 Nac's Cannondale Moto - sold
87 310R - mx equipped - sold
YFZ 450 - mx equipped - sold
416ex - mx equipped - sold
'86 250r - mx equipped - sold
'87 250r - mx equipped - sold
250x/350x hybrid - sold
lt250r quadracer - stolen years ago