The verb form pueda is in the present tense, while pudiese/pudiera are in the past (technically, pretérito imperfecto). They are not interchangeable. Which one to use depends on the point of view of the speaker in time, and the proper temporal correlation with the main verb (the subjunctive is almost never used in a main clause; it appears in subordinate clauses).
Let's see your first example:
Pero su presidente, el polaco Michal Kurtyka, ha decidido adelantar los contactos entre todos para que la decisión se tomara este viernes, como así ha ocurrido.
This is a rather complicated setting. The perfect ha decidido shows an event in the very recent past that has persistent effects now; it's a past that continues into the present. Without further information you would then use a subjunctive in the present (para que la decisión se tome). But in this case, just after that we get este viernes, como así ha ocurrido, so we know the whole thing is already in the past. Therefore the subjunctive verb goes in the past as well: tomara. Suppose the Polish president had done something to make the others decide not Friday (today), but next Monday; then the example could read like this, maybe:
Pero su presidente, el polaco Michal Kurtyka, ha decidido adelantar los contactos entre todos para que la decisión se tome el próximo lunes, como se espera que ocurra.
(Note that tome and ocurra are present tense, even though they refer to the future. This is because the future subjunctive is almost totally obsolete in Spanish, so we use the present instead.) Even so, this is a tricky one, and some speakers would still use tomara (past) instead of tome (present) in this case, since the president's intent is in the past.
The rest of the example (with renunciara) follows the same idea. It's something that happened last Wednesday, so the verb goes in the past.
Next example:
"Si alguien me pudiese encontrar algún transporte, le estaría agradecida."
This is a combination of two things. First, the usual correlation between tenses in this kind of conditional sentences goes like this:
- si + past subjunctive + (entonces) + conditional
- si + present subjunctive + (entonces) + future indicative
Your example uses (1), but you could also use (2):
"Si alguien me puede encontrar algún transporte, le estaré agradecida."
The meaning is basically the same, but (1) is more hypothetical, while (2) is more definite. The real difference is of tone: (1) is more tentative and therefore more polite.
