Is it permissible to choose the easier opinion (not making up the prayer) regarding the ruling on one who prayed in a state of major ritual impurity (janabah) out of ignorance, and is that considered following dispensations (rukhsa)?

We hope that there is no objection for you to adopt the easier opinion, and this does not fall under the category of "seeking out concessions" as long as it is not your usual practice. This is unless you have come to believe the opinion of the necessity of making up the fasts, then abandoned it and adopted the easier opinion.