Regarding part A of the question, the Beis Yosef in Orech Chaim 102 s.v. כתב cites the Mahari Abuhav as saying that one needs to wait even if the person behind them started after them.
As far as the people in front of you (the person waiting), R' Shlomo Zalman Aurbach (Halichos Shlomo Hilchos Tefillah ch. 8 os 47) writes that the shechina is still "present" even after you finish until you take your steps back, which would imply that the person in front is also "stuck." In os 34 he writes that if the person behind you is finished, you could go back for mitzvah purposes. One example of mitzvah that he gives is tachanun, so that would mean that the person in front would not be stuck once the congregation is moving on to tachanun.
R' Chaim Kanievsky, in Siach HaSadeh vol. 3 berachos 10a writes that it is OK to go back if the person behind you is finished as long as you avoid the 4 amos of the person behind him who is not finished.
(Seemingly, this should depend on if the reason one cannot pass someone praying is due to interrupting their concentration (Magen Avraham O.C. 102:6) or interrupting between them and the shechina (Chayei Adam Klal 26, #3). If it is a concentration issue, once they are done there is no more issue. If it is a shechina issue, then the problem persists as long as the shechina is still present.)
The Kesser Rosh of R' Chaim Volozhoner in Hilchos tefillah number 33 says that it is אסור to take the three steps and there isn't an issue of יוהרא (looking haughty) in waiting.
Regarding part B, the same source says that while you are waiting you should say techinos and bakashos, and learning is also allowed. I would assume techinos extends to the continuation of davening in most situations. Certainly tachanun. R' Mintz (the posek in Ner Israel) told me that this would include putting one's head down on one's arm, but not sitting.