Ok, I think we need to first call FSSJ exactly what it is.
Physical Conditioning
Goku and Gohan's training consisted of the same time of training endurance runners do. The point is to maintain elevated levels of exertion so that they become your new baseline. For example, endurance runner have significantly lower pulses then the general population. The reason is that their endurance training consists of running hours at a time so that their physicological running state is similar to an normal average person. In this way they are able to concentrate their energy on the practice of running during a competition without using as much energy to maintain that elevated state during running.
The second time of conditioning is speed training. This is where you push for faster and faster speeds, even at the expense of energy depletion. Because these are burst or sprints, energy consumption is not the focus but maximum power output is.
These types or training do not automatically guarantee gains in the other although they can. Because you can run a 5 mile marathon at 10mph, DOES NOT make you immediately capable of running 400 meters at 25mph and vice versa.
That said you evidence that FSSJ replaces SSJ Grade 2 makes little sense as they have two completely different functions. FSSJ improves endurance and efficiency. Grade 2 improves max output at the expense of energy and moreso with Grade 3.
To answer you question on why they didn't use it, it depends on who you ask and what you consider canon as Vegeta appears to use it in the anime as a boost when needed.
https://www.youtube.com/watch?v=ZKyG8EXh7D8
Since I will hear none stop arguments about using the anime we will ignore that for now for the sake of argument.
I will use a marathon as an example. As a runner you do not run at maximum speed straight out the gate. In the case of the Z fighters, using Grade 2 does nothing but expend precious energy without knowing how long the fight will go on and should only be used if approaching the finish line. Smart runners conserve energy until the finish line is within reach. In the case of the Z fighters vs Cell they were so far from being able to take on Cell that getting the small boost from Grade at the expense of possible exhaustion could have literally killed them as they wouldnt be able to defend themselves as easily. In this way saying it wasnt used. (eventhough it was in the anime) is a poor excuse as it wouldnt have made sense to open themselves to the possibility of exhaustion and death.
Now on to the power levels of the grades. We know that SSJ is supposed to be 50X base and SSJ2 is 100X base. If they other forms fall somewhere in between as you say then I would estimate SSJ grade 2 x75, SSJ grade 3 x90.
You can see with each grade level there are diminishing returns and they forcibly increase their energy output. Vegeta and Trunks focused on making these forms output more power but at the cost of energy and were not sustainable and to only be used in short bursts of fights.
Even Cell senses Gohan's ki and states that as Gohan is powering up to SSJ2, Trunks used a similar technique which didnt end well for him. Cell is detecting the same type of power spikes from Gohan as he does Trunks.
I'm not following you on the Frieza thing because I thought we were talking about SSJ.
Anyway FSSJ is nothing more than a conditioning state for endurance in the SSJ form and normalizing the SSJ as easily as their base, a trick most professional athletes especially running and cyclists use. While this
can have the added effect of making them faster/stronger it is not the primary purpose or result of this form of training unlike Vegeta and Trunks training.
