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1) You have 8V worth of LEDs but the output high voltage of the 555 is 7V with a brand new 9V battery which drops lower and lower as it is used. So the LEDs will not light.
2) The datasheet for the 555 (NE555, TL555 or LM555 shows the parts values for any frequency.
3) Every IC circuit powered from a little battery needs a supply bypass capacitor (use 100uF).
Yes. But the output voltage loss is even more for sinking.He doesn't source the LEDs but sink.