Addressing one by one ( I don't mean to impose my views on you, I am just trying to present some points) :
1) If the same loco has to be used after reversal, then it involves the following steps ( excluding the final attachment of the loco to the train):
a) After loco is detached, the new crew has to take over the locomotive and perform the usual diagnostics (brake power etc) before starting up the locomotive. Usually takes a few minutes.
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Controllers have to wait for line clearance and set the points, and block the respective lines. This process is usually the time consuming one, and at busy junctions, can cause a lot of delays.
c) Once the loco starts the process of switching ends, the crew has to switch cabs and possibly swap pantographs too twice, once for each reversal. This again eats up a few minutes.
d) The loco has to travel from one end of the station to the other.
If a fresh loco is used, step a is already done by the time the train even arrives. step 2 is no longer necessary and at busy junctions, this means a lot as this step is the most time consuming step. And step 'c' is also not necessary as the loco doesn't have to reverse multiple times. This saves quite a lot of time. At busy junctions like BZA, a new loco is attached within a few minutes for many trains that are having reversals, even at peak hours.
2) Earlier, when E-locos were not so abundant, pretty much every train having a reversal used the same loco after reversal. But as the number of locos started increasing and the availability increased, more and more trains started getting loco change when reversal is required. As KGP trip shed has an abundance of SRC WAP's now, they are able to provide a new loco here and they are preferring that, as it's convenient and faster.

And still 15mins halt is not enough to complete all these steps right ?? Earlier when the halt was 15mins all the above mentioned steps were still performed within the stipulated time duration and now when still the halt is of same 15mins duration then why can't the all the conventional steps be applied ?
Then I must wish KGP halt should b withdrawn like in the case of Odisha SK and Utkal exp if possible but it will drastically reduce revenue and food loading at KGP ... but food loading can be done at BLS also as it too has a base kitchen though not inaugurated yet..
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I conclude that many people (who don't even have anything to do with BBS Raj and ECoR and KGP authorities) literally don't want this train to be hauled by P7

It's not about them not having enough time to do all the steps, but it's about them doing it in the most convenient and efficient way possible. Using a new loco instead of reversing the loco is more convenient for them and in cases where it is a busy junction, it will also not affect other trains and other routes and controllers practically need not be involved at all in the loco change. And it's not an isolated case for BBS Raj and KGP, but it's the same at every major junction. If the station has an associated trip shed and if locos are available, a loco change is always preferred over a loco reversal. And in this case, KGP staff have no reason to ignore the easy process of loco change and do an reversal instead, as it doesn't make any difference to the operation of the train itself. They...
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are just following the more convenient and efficient step.

The concept of prestige based on loco is just for us rail fans. Each of us would consider it as a prestige if our favorite train is hauled by our favorite loco. For IR, all locos are the same and allotment is always done based on operational feasibility. The authorities have nothing to gain by not allowing a particular train to run with a particular loco.
For example, as you pointed out that all Raj trains seem to be getting P7 as a standard link, it's not because IR feels a P7 is a 'prestigious link'. The reason is that LHB rakes require more powerful compressors in the locomotive to be able to operate the brakes and other equipment. The 3 phase...
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locos like P5 and P7 had these higher power compressors right from the beginning. The older generation of WAP4 locos did not have sufficiently powerful compressors for this. That's why, when the Raj's stopped getting WAP5 locos, they were changed to use WAP7 almost exclusively, except for shorter rakes and less aggressive runs even though the P4 was powerful enough to maintain the schedules. Over time, many P4 locos were upgraded with new equipment to handle this problem ( like how SK always runs with WAP4 even though it has an LHB rake) and are now being used for LHB rakes more.
Anyways, I don't mean to imply that BBS Raj does not 'deserve' a P7, I just mean to say that all the locos are the same for IR. They'll use whichever loco gets the job done.
My apologies for the long chain of arguments. :(