There are four cables branching out of the battery +ve, three going to the alternator, and one to the starter.
One of the charging cables is the car's original cable, and the two added on seem to be in decent shape.
I added a grounding wire between the cylinder head and the bulkhead.
Visited Fareed Electrician based on @capsat recommendation. They didn't do anything except take the alternator out and have me visit another shop in Magazine lane:
They determined my "kataut" was faulty and replaced it. Tested it using a motor that spun the alternator and showed 14V unloaded. When I asked them about why they don't measure loaded voltage, they gave me the usual - "tajurba," "saalon se kar rahay hain," "befikar rahain," etc.
An ammeter and a few cheap lights in series would have done the job, but not having it gives them plausible deniability for their crappy junkyard alternators.
Alternator installed back in the vehicle. 13.9V unloaded, 12.3V with AC and lights (55W H4 bulbs) on - measured at the battery.
Fareed said "itna hi hota hay, sahi hay." Said there was no need to add grounding, and that the alternator was working when "slow," so it's fine. Also said that if this alternator didn't do the job, larger ones were available and that "fitting" karni paray gi.
The car still hesitates under boost, although it's slightly better.
The max rated amp output is not at all the sole point of concern, it's the voltage drop under load. Are there any competent electricians that actually understand electricity?