Wrong, U2s were sent to picture nuclear facilities and collect those material samples in the air around nuclear test site. By analysing these samples, Americans had pretty good idea of the material used in Chinese nuclear bomb. On the other hand, the major way to estimate the scale of yields is seismic stations instead of U2
First, India's 1998 tests were claimed as big as 55k in total (43k, 12k, 0.2k), only one of three was sub-kiloton as you suggest here;
Second, the wildest estimation of these tests is 25k, which means even India uses every possible method to cover her tests yields, the result was still far away from her claim.
By your own government, among 3 tests in one day, only one was sub-kiloton test. The one disputed mostly was 43k device. So, sub-kiloton has nothing to do with the discussion here.
First
Yields can be estimated not only through seismic means, but also by other methods. The
other methods are based on the radionuclide analysis of the nuclear byproducts of the explosion (by radiochemical methods) and measurements of the speed of the shockwave generated by the explosion
in the surrounding rock (hydrodynamic methods).
US govt never uses radio nuclide or hydro dynamic methods to test yields outside US so there goes all your theories outside the window.
Second
The Mb ( Body waves ) and Ms ( surface waves ) data points are used from the NEVADA TEST Site where US did its test which has a different geology than pokhran.
Third
Pokhran test sitegeology has some similarity (probability=.77) with Kazakhstan test site used by the Soviets for which US always assumed the yield and gave a disclaimer .
Fourth
Mb is calculated as m b = log (A/T) + B.
Ms is calculated as Ms = log (A/T) + D,
where A and T are the amplitude and the period measured off long-period vertical component
seismic recordings in nanometers and seconds
B is a distance-dependent correction term that compensates for the change of P-wave amplitudes with distance and D is a distance-dependent correction term for Rayleigh waves.
and yield of a nuclear test is measured by Mb = a + b log10 Y where Y=yield : Here constants a and b vary with geology of the earth surface.
What value did they use for constants a and b cause they have to examine the geology of pokhran to calculate yield. What is the value of Mb used by westerners.
Fifth
Have they factored in the interference pattern of the 2 simultaneous tests of 43 kt and 12 kt during Pokhran 2 as both Mb and Ms will deviate from log normal pattern. By the way it was a destructive interference.
Sixth
The measurement of Ms requires a larger event, because Rayleigh waves are small for nuclear explosions. For explosions below 50 kt Ms may be missed altogether at tele seismic distances which was exactly what the westerners did they used the Ms from tele seismic distance of their seismic arrays to measure without factoring in the a , b and the Mb of the test
These are some of the basic points raised for seismic testing of yield which itself has a discrepancy of 30 % and no radio nuclide test or any arrays in India is like taking a wild guess which has no place in scientific experiment.