Q. 3- (See prior questions – If I don’t have a Shlisel Chala) Can I just buy a regular chalah and stick a knife into it, then introduce the key and the re-bake on a fire the opened incision, as we do with a shemura matza that is slightly cracked at the edges?

A.(See last question) It would seem that for this type of minhag the procedure you suggest would suffice. The above method is indeed used by many for fixing matzos that are missing a bit at the edges and making them complete (Seder Haoruch 22: 4 quoting Horav Eliashuv zt”l, Ma’adanei Yom Tov 4:p.698)
Horav Yaakov Hirschman Shlit’a and other Rabbonim I consulted came to the same conclusion.
However, you should be advised that Shlisel Chalah is available ready-made in some bakeries.

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