Aditya ji, my inference is slightly different. Please refer to attached map
My take is based on following assumptions -
1. TSPAF had 4 group of 4 fighters each - a)4 M -3 extreme south (facing mid Punjab), b) second group (main AG component)- 4 JF-17 and 4 M-V slightly north of it (facing our north Punjab), c) 4 F-16 over mangla dam - facing Jammu - Their main cap (A-A role) and finally d) 4 F-16 further north (possibly both AA and AG role and the commander of the mission here, alpha -3)
2. Facing A and B group were 2 M2000 (upgraded ones) - That attack from TSP did not go anywhere because of M2000 presence. JF-17 and M5s did not press in and possibly missed targets (my assumption still is perhaps they were not targeting, as we had not targeted their military, it was signaling, but even if they wanted, the attack would have failed as M2V had to break lock)
3. The C group of 4 f-16S(along with D (4-f16s), D not perhaps fully engaged) were engaging 2 SU30MKI.
4. As you rightly point out at this time the D group of F-16s start their A to G run (the C group is pure CAP)
Now here things get interesting. 2 Mig 21 shows up suddenly on TSPAF AWEC planes (or even F-16), as soon as they pull up from the shadows of pir panjal ranges. The two Mig 21s are apposite alpha 2 and alpha 3 of D group of F-16. The 4 f-16s from the D group, their alpha 1 is most south, followed by alpha 2 slightly north of it (apposite Abhi), alpha 3 F-16 is opposite wingman of Abhi (and he is most east as he is initiating LGB attack, nearest to the border), alpha 4 is farthest north.
TSPAF AWEC allocates Abhi to Alpha 2, and his wingman to Alpha -3. Alpha 3 breaks LGB attack and engages Abhi's wingman through AMARAAM. The wingman does the tactical maneuver, gets out of NEZ and puts AMARAAM in chase mode and ultimately escapes.
Meantime, Abhi also targets Alpha -3 who is busy shooting at his wingman, is most east and near the border (alpha 4 would be his wingman, while alpha 1 and 2 of TSP formed another pair) and towards his north and perhaps at his 2- 3 o'clock (the perfect place for a R-73). R-73 gets a lock and and shoots down F-16 (APHA-3), who may not have known till the last that there was a R-73 heading its way. As far as he knew, Alpha -2 was keeping the other Mig 21 busy.
The TSPAF F-16 (alpha 2) was some half a minute to a minute late, it reaches the scene, while Abhi is exiting the area after a successful kill, he sneaks close enough and perhaps in NEZ for AMARAAM and fires. Even though ABhi is in tail chase mode, AMARAAM reaches him.
If the above is true, we were indeed lucky that day.
The MIG 21 did not go after the C group (that was pure cap and engaging SU 30 MKI, they were not A-G, group D was, and they could have caused causality if they were not just sending messages, but Mig 21 spoiled their plan. I think the ground control would have read that and vectored Mig 21 correctly).
Also both Mig 21 engaged the same F-16 (alpha-3), if Abhi went after alpha -2, he would have done the same maneuver as his wingman, both would have disengaged, and dodged the AMAARAMs. In this case, no loses from either side. The Alpha -2 of TSP was not the real hero, in spite of shooting the M21, he was slow, and because of that, F-16 was lost.
At abhi's 3 o'clock and downed F-16 9 o'clock, both planes were in the blind spot of each other radar (no radar looks at 90 degrees). But the Hemet cued R-73 would have picked F-16 as it came to it's 3 o'clock. It was a fight readymade for M21 and Abhi made sure that it was executed to perfection.