Was Jacob a true believer when he obeyed Isaac's command?
No, Jacob was not a true believer when he obeyed Isaac's command. Even in his unbelief, he was still under the auspices of faith, yet he was not a true believer. His obedience was enough that Esau could see he had obeyed his father and gone.
Les teaches that Jacob obeyed his father Isaac, and even in his unbelief he remained under the auspices of faith, but he was not a true believer. His obedience was enough for Esau to observe that Jacob had obeyed his father and departed. Esau then tried to do two wrongs to make a right by marrying a daughter of Ishmael, but that approach does not work. The context stresses that Jacob's outward compliance did not equal saving faith at that point.
When Esau saw that Isaac had blessed Jacob, and sent him away to Padanaram, to take him a wife from thence; and that as he blessed him he gave him a charge, saying, Thou shalt not take a wife of the daughters of Canaan;
See the rest