How can someone who is makpid on gebrochtz cook food like knaidlach or other gebroktz on sheviyi shel pesach for shabbos which is acharon shel pesach this year? how can the eruv tavshilin help?
I found an interesting Psak Halachah in the Lu'ach of Rav Tukachinsky that is relevant to your question. It says that an Ashkenazi - who does not refrain from eating Kitniyos on Shabbos that follows Shevi'i Shel Pesach - can cook chickpeas on Shevi'i Shel Pesach/Erev Shabbos for a Shalom Zachar that will be that Friday night. The reasoning is that it is possible that a Sephardi may visit him on Friday and eat the chickpeas, so we say "Ho'il" and his Eruv Tavshilin is effective.
It would seem to be a Kal v'Chomer to say that this Psak applies to Gebrochts (one might have a guest of Friday that "Brochs" and can eat the Knaidlach), given that Kitniyos is a universally accepted Minhag among Ashenazim whereas Gebrochts is not.
Chag Kasher v'Sameach,
This is not a Psak Halachah