As far as I understand, Brezhnev was told of the factionalism in the PDPA in April 1979. By September that year Taraki was dead and Amin was in power. When the Soviet’s invaded in December they had Amin executed, since they viewed him (rightly) as an ineffective client. I think you’ll agree that it’s quite strange that Amin would request help and then be summarily shot by his saviours, but then regime change is never pretty. Brzezinski’s push for arming the Mujahideen came almost immediately after the Soviet occupation of Kabul in December. In March ‘79, we know now that a number of secret meetings were held in Pakistan to this effect. However, the ISI has been attempting to destabilise a soviet dominated Afghanistan since ‘74 at least, they were hostile to Daud Khan’s fantasy of a united Pashtunistan and all the implications that had for Pakistan’s territorial integrity. What the CIA did initially was to merely capitalise on preexisting gun running operations across the Durand line set up by the ISI and what’s more the initial sum of money provided by Carter to the rebels in late spring of ‘79 was intended to be non lethal according to Steven Coll’s exquisite book on the subject. So in short, the PDPA was so torn by factionalism that whichever side called for soviet assistance to “stay in power” was clearly not in power at the time(amin’s execution), and the decision to arm the mujahideen with lethal aid at any rate was taken post-December 1979, in the aftermath of the invasion.