About your questions

1 - Marcelo Caetano was percieved as a weaker leader compared to Salazar, and with continuing the colonial war pretty much set up the stage for discontent to grow even more.

2- The soviets had no active (as in, sending explicitly troops/ammo/supplies) aid in the Carnation Revolution. The portuguese communist party (PCP) had its support though, and through the mobilising of the army that was fed up with the losses of the colonial war and the overall state of the country, they managed to amass support within it

3 - As for why it didn’t succeed, it would have been basically near impossible because even though Portugal was a colonial power, it was one of the lesser ones and, being sandwiched between Spain’s Franco and the overall western Europe, and the US, it would have never been allowed for it to go the communist route. You can look at this comment of mine and thread as a whole for some insights as to why the revolution ended by 25 of november.

https://hexbear.net/comment/3968448

As this was happening, the leftist parties besides PCP were having a bit of a meltdown. The socialist party (PS) that during the dictatorship was advocating for socialism, took a turn after the revolution and became a standard european social democratic party, even as far as going about the blairite third way style of politics. Mario Soares (their leader at the time) pushed for integration with the US and western europe sphere of influence, which meant staying under the capitalist status quo. Besides PS, there was also PCTP, which was a maoist party, and they also disagreed with both of them, further creating discord between the left. Of course, with all the this the right wing parties took advantage of it and pushed for a “stabilization” of the country that culminated in the 25 of november counter-revolution. Here is some insight on it link ( its in portuguese, but put it through a translator like deepl and it should give you the gist of it)

Hope it helped

Edit: Here’s a good book tituled The truth and lies of the revolution of April (The counter-revolution confesses itself), written by Álvaro Cunhal, the historic leader of the PCP (in portuguese) link

It would have never been allowed by the US and the west.

https://www.workers.org/marcy/cd/samport/port/port02.htm

An interesting read and also gives a little more context. There was literally an US aircraft carrier at the gates of Lisbon, in case the country had the intention to become a communist one.

The guns and armor of the American aircraft carrier Saratoga stood out menacingly within sight of Commerce Square, opposite where most of the workers had assembled. This had to arouse the greatest indignation, not only from the demonstrators but from all Portuguese workers, especially when one remembers that the CP itself has stood out so long and undeviatingly against NATO. Yet one must wonder-how could the Armed Forces Movement agree to the participation of Portuguese naval units in an exercise which involved simulated bombings of central Portugal? And how can the Provisional government at this very late date still be a member of NATO, especially when the Pentagon has contemptuously barred the Portuguese armed forces from any so-called secret NATO material?