Will; I guess you have answer to your questions by now, but in case, here is more help if needed:
1- LINK to download manual for 400D
in case you have lost or misplaced yours.
2- For M mode (Manual exposure) turn to page 71.
In short you adjust Shutter speed by moving Dial on Right Top of your camera.
You adjust the F stop by holding down the Av+/- bottom and turning same dial as above. Av bottom is the top bottom on the right of preview LCD.
3- AEB is discussed in manual's page 76.
It is not a must. You can start with Rule of 11 and then adjust accordingly.
4- Rule of 11 is a deviation from "Rule of 16" know well by all old photographer who are familiar with cameras that used to lack exposure meter so had to make a guess and chose their own exposure. Rule of 16 states; If you are shooting under a normal sunny day, if you use F 16 (Thus rule of 16), and film with ISO 100, your right speed is one over ISO i.e. 1/100 or for those days camera, closest shutter to this, 1/125.
Now with over cast sky, you decrease one shutter stop, and one F stop or two of each. Then for shadows, indoor, you add two more and list goes on.
Here you use F stop 11, ISO 100 Speed 1/125 (Rule of 11)
5- Remember if you are lucky enough to spot the first night (Or last) moon (That dosn't last much and you need to look low near horizon on the west, one good example is/was tonight, Oct 1st), it has brightness intensity of %6 or full moon that roughly increases by 8 percent a night till night 14 when moon is at full brightness. This rule is for mid cycle so you may want to decrease of increase exposure equivalent (EV) as needed. It is a lot easier done than said. Just start with same ISO 100 F/11 shutter speed 1/125 (If your set up is not stable enough, decrese one from F and add to shutter or do it to multiply that is limited by your working max wide F stop, remembering that if you use 1.5x teleconverter, you have 1 full stop lower thatn shown on dial of lens), and check the exposure on LCD. If you are good with histograms (On X axis far Left is too dark, far right is too bright and in between is just the right exposure and tallness of Y axis is pixels harboring that brightness). But again reading histogram is not a must. Just look at the picture if it is too bright or just enough bright, you are over-exposing. Decrease your selections. If it is a littler darker than what you like to see, It is perfect. If too dark, then you know what it is. Alternatively, you can bring the memory card back to your PC and check the magnified images once downloaded to see critical focusing, in addition to right light and go back and continue as needed.
Except for cloud, let me know if you can make any other excuses!
As for cloud, well cross your fingers and ask the higher authorities
One good advice here, You can get closer to higher authorities at higher altitudes, so if you have mountainous location near by, don't hesitate to drive there that not only you have a better atmosphere for picture taking (Lesser smog, pollution) but lesser chance of low clouds, fog etc. Being close to cost myself, I see a significant change when high in mountain. Fortunately for me, the web cam at place of my interest (The place you can see Milky way on my gallery) on the internet, can give me a live view of place that refereshes every 5 minutes so I can deside whether a trip is worthy or not, with a lot of certainity. You may want to check such things in your neighborhood.