If Pakistan does anything even remotely similar, it will be treated as proliferation and will likely invite sanctions. Bangladesh also is signatory to NPT as a non-nuclear state. Try to withdraw from it and there will be sanction consequences on both of the states. At BEST bangladesh can argue to host Pakistan controlled nuclear weapons but to actually have nukes with Bangladesh forces is essentially inviting sanctions on violation of NPT.
I highly doubt that powers-to-be in Pakistan will even remotely consider to sanction and to submit itself to a complete nuclear annihilation in a war they might not be party to. Remember, there is no NATO kind of Article 5 between two countries and neither Pakistan is strong enough to actually put together anything similar.
Besides, there is nothing of value for Pakistan and its decision makers in this.
Lastly, I highly doubt India will threaten to use nukes first on Bangladesh without a provocation of nukes or WMD. In a purely conventional war, India can wipe the floor with Bangladesh. So why will it escalate? Without nuclear threat, any kind of treaty or nuclear weapons sharing is useless as a means of deterrance.
NWS is not seen as proliferation funny enough, the US and Russia BOTH do it with respective partners.
As for the mutual defense, we are talking hypotheticals, obviously Article 5 doesn't exist, I'm talking of a scenario where is gets created.
The conventional warfare disparity is precisely the reasons for the nukes. You are imagining a scenario where a country acquires nukes to deter the nukes of another, but thats not the scenario here. The Pakistani scenario for Nukes are there precisely for a 1st strike deterrent in the scenario where a conventional arms threshold is crossed. Pakistan does not have a "no first use" policy by design, b/c under the Pakistani doctrine it will likely be the first to use nukes in any significant conflict, for the point of acting as a deterrent, so as the other side know they they can't "wipe the floor" with anyone in conventional warfare, without risking themselves getting wiped out.