While there is no "Magisterium" per se regarding the subject, if you don't count the 2005 ruling from the Conciliar Vatican, it's considered "common teaching" that the condition of praying for the (objective and enumerated) intentions of the Holy Father continue to be satisfied even during a period of sede vacante, and there's no available text that would contradict this notion. Regardless, we receive all indulgences at the mercy of God anyway. If God does not grant them to us, then glory to God. Similarly, for those of us who have consecrated all these to Our Lady via the method of St. Louis de Montfort, they remain at her disposal anyway and we no longer have any claim in justice to them either.
As far as the changes, Montini got rid of all non-plenary indulgences, or, rather, reformulated them so that the Church throws in a "matching contribution", as it were, doubling the intrinsic value of the works or prayers ... which is not very consoling
Nevertheless, I am not entirely opposed to this change, other than that I consider it invalid due to the non-papacy of Montini ... since I've known a fair number of people who became neurotic or obsessive about "compiling" indulgences ... leading to an almost Pharisaical and certainly legalistic perspective on sin.
As for the plenaries, the one devotion that was not changed, so that it was the same before and after Montini's changes were the Stations of the Cross. It's one of my favorite anyway, and it's the only devotion I make with a view toward receiving indulgences, even though it's not the primary motivation.
You can't go wrong with the Stations. I also do not like to use any specific formula, nor are you required to, nor are any specific prayers required. One need only physically move (if possible given logistics, your own mobility etc.) from one station to another and at each station contemplate the events of the station. There's no time requirement either. I find that, if I'm in a hurry, I can make a meaningful and beneficial Stations in roughly 7-8 minutes, spending about 30 seconds meditating upon each Station.