Reply To: Skipping in pesukei d'zimra

“walking into weekday arvit while everyone else is davening Sh”E”

Well, the poskim discuss this case explicitly and say that one should skip Krias shema and its blessings, and recite the silent Amidah together with the tzibbur, and make up what he skipped later. [This is assuming that he won’t be able to get a later minyan.] However, some poskim do not agree with this [Arizal, Rav Auerbach].

“davening mincha by themselves while everyone else is singing L’cha Dodi”

This is also discussed by the poskim, who write that if you start Mincha before the tzibbur was mekabel Shabbos, it’s OK, but once they were mekabel Shabbos, you should not daven the weekday Mincha in their presence [this is a chumra].