multivibrator frequency calculation
1) For this circuit to work properly the collector resistors need to be significantly smaller than the base resistors, just like you found. This is because the main time constants in the circuit should be C1 with R2 and C2 with R3. If you make the collector resistors too high in value these will no longer be insignificant, they will also influence the charging and discharging of C1 and C2.

For proper operation the voltages at the collectors should toggle almost instantly, or more accurate: siginificantly faster than those two time constants (C1*R2 and C2*R3). I would always use at least a factor 10 between the collector and base resistors so

R2 >> 10 * R1  and  R3 >> 10 * R4.

Usually R1 and R4 both 1 kohm should do the job.

2) I think the 1 Mohm base resistors are playing tricks on you here. Due to their high value you will get very small currents and leakage currents probably start playing a role. I would try again with all resistors 10x lower and all capacitors 10x higher value and see if that matches your calculations better.

3) You only reaching 11 kHz has nothing to do with the transistors. It's the component values you're using. You need to speed up your circuit by lowering the impedance levels. This means making the collector resistors as low as possible. Of course too low will result in too much collector current and too much dissipation so keep track of that. Probably 200 ohms is the lower limit depending on the supply voltage.

Since you want the base resistors also as low as possible but not lower than 10 times the collector resistors, you could use 2 kohms. Then for C1 and C2 the smallest capacitors you have, my guess is that 100 pF is a practical lower limit. That should oscillate at a couple of MHz ! I was too optimistic. With Rc = 220 ohm, Rb = 10 kohm, C = 390 pF, T = BC547, Vcc = 5 V I could reach 200 kHz on a breadboard. It did not always startup immediately indicating that the loopgain is a bit low.

Sidenote 1: You don't need really need R5 but if you insist make it at least 10 times higher in value than R4 because a low value will prevent the collector of T2 reaching the supply voltage.

Sidenote 2: I see no current limiting resistor for the two 1 W LEDs. I would add them because LEDs need some form of current control.