Shouldn't reaching an octet be any atom's "goal"?

However, I've recently learned about cases that are either expanding octets, or have lesser than "enough" electrons for an octet abiding. e.g.:

S in Sulfur hexafluoride (Expanding octet with a $\text{3d}$)



Source

B in Boron trifluoride (It's "hexet", instead of octet)



Source

However, both $\ce{SnCl2}$ and $\ce{SnCl4}$ are existent. The latter is explainable with octet, but not the former. Surprisingly, $\ce{SnCl2}$ is more stable! 


How?
Why is the phenomenon happening?




Dave pointed out that the octet nonabidingness is happening in the vapor phase. So I redirect the question to ask about the vapor phase; since that's what I'm looking for.


Question: how to calculate intersection time and place of multiple moving arcs
Answer: If your bodies are subject to complex orbits it may well be difficult or impossible to solve using a closed-form expression (solve an equation to get the desired results.

One approach would be to perform a mathematical simulation over time to arrive at a solution.

In your case, a body leaving A at a time t0 travelling in a straight line with velocity v will be somewhere on the circumference of a circle or (sphere in 3D) with radius vt (t=time since leaving). The centre of the circle will be the position of A at the time of launch (Apos(t0)). You can imagine this circle like a ripple on a pond getting bigger over time)

Assuming you can calculate the position of B at time t+t0 (Bpos(t+t0)), you can create a simulation with a loop to find out the earliest time that the growing circle intersects B's path

searching=true
t=0
while(searching){
    if (distance between Apos(t0) and Bpos(t0+t)) &lt; v*t then{
        searching=false //found a solution
        target_position=Bpos(t0+t) // can be used to find direction to head in
        time_to_reach_B=t
    }
    else
        t+=delta_t //a small time step (smaller = more accuracy)
}


You will of course have to add code to prevent an infinite loop in the case that there is no solution (or no solution to be found in reasonable time).

Depending on the complexity or B's motion and the value of delta_t, this algorithm is not guaranteed to find the ideal solution in every case, but should work well for the vast majority of cases.