Q. We have a minhag in our family that we bake a chalah for the Shabbos after Pesach and we place a key inside that chalah, (shilsel chalah) This year due to a simcha my wife will be travelling and not with the family. A number of questions.
Q1- What is the source for this minhag?

A. Although this tradition is mentioned in many seforim, (Ta’amei Haminhagim 596-7, Minhag Yisroel Torah 2: 493, Milin Kadishin p. 305, Imrei Pinchas 298, Nitei Gavriel Pesach 3: 39: 1, Lashon Shel Kedusha p. 123, et. al.) mostly dealing with Chasidic minhogim, they usually quote as the source the Ohev Yisroel (Likutim p. Shemini),
They provide different reasons for the tradition, such as: After Pesach, all Holy Portals that were opened on the Yom Tov of Geula and redemption, are now closed. The key is a remez and an allusion that they could still be reopened until Pesach Sheni (Ohev Yisroel ibid.) Another rationale given is that at that up to the time, when the Jewish Nation entered the land of Israel, they still enjoyed sustenance from the Heavenly Bread. After Pesach, when the Mon became unavailable, they required the Mafteach or key of Parnassa (ibid. see Talmud Ta’anis 2a)) They also explain that the key alludes to the daily opening of the Fifty Gates of Kedusha during the days we count Sefiras Haomer.
An interesting insight is mentioned in Ikvei Binyomin (Acharei 18: 3) on the posuk “Do not follow their decrees” meaning: During Pesach we become totally separated from the rest of the nations by the extraordinary prohibition of not eating and owning chometz. After Pesach, we regain permissibility and there is risk of unacceptable intermingling with them and their “chometz”, so we need a key, not to open but rather to close.
The above gains value when we consider how much has been written in regards to the possible (but unlikely) correlation of this tradition with the Christian Easter Breads manufactured in the form of a key and a cross. On Easter, the Christian holiday which celebrates the idea of resurrection, the key shaped like a cross is placed inside a rising loaf.
There are also different ways and means as how to comply with this tradition. Some, as you mentioned, introduce a key into the bread batter, others shape the chalah in the form of a key. Some (Yismach Yisroel – Tazria, Minhag Yisroel Torah ibid.) would bake it round or change the colour, as a remez to Geula (geyler or yellow.)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a