it cant be IAC as you say it takes longer to start as well. try this once its stalls and takes longer to start floor the accelerator while cranking but immediately remove when engine starts, if flooring the accelerator while cranking makes it easier for it to start compared to no accelerator then you should do two things
goto another toyota dealership and have them look at the fuel system from start to finish again, check fuel filter again fuel pump, fuel lines and rail pressure and injectors, it can be that the toyota dealer you already went to caused the problem themselves and are now trying to cover it.
2) go to any good EFI electrician ask around in pakwheels for who's better so that bad ones dont cause more damage with their wrong test lamp techniques. go over the wiring in the engine bay especially around the engine block and check coil packs etc.
because you have done so many things your are probably getting ocnufsed and trying hit and miss technique now. it will help to go over the fundamentals from a different person once again. shouldn't cost much.
about the fuel system treatment i did once use liqui moly's in my mini pajero it was fine made a slight difference. the thing is after pouring the fuel system treatment you have to run the car hard on open roads not on short distances and make sure that the engine gets upto temperature and also a little hot as all the cleaner does is soften slightly all the debris in all the fuel ways so if you do not run hard and hot 1) it wont make a difference and 2) the debris can become stuck in the fuel lines as well even though you did well to change the fuel filter after running the treatment.
By the way it is not necessary a wrong sensor will have stored code while scanning or check engine light will be on, sometimes sensors can become out of spec and that you can only find by reading the values from the output, a filing MAF need not necessarily store a code or throw a check light if its out of spec, one can judge better from looking at its fluctuating value from the readings.
but thats why i told you to check while pressing accelerator while cranking as that will rule out sensor and will points to fuel or spark issue.
its possible that combination of fuel system treatment and pure hi octane has done the injectors for good causing restricted flow at lower rpm and while cranking.