I found the write-up impressive.
The amount and breadth of research is impressive. As are the ideas for skills.
One comment I have is the profession as described seems to be a combination of engineer, alchemist, trap expert, siege weapons expert, inventor, and several more. I'm not sure if any medieval age person would be able to acquire so many diverse (though related in some way) skills and knowledge in a lifetime.
In ancient times, a skilled and experienced engineer would already pack long years of experience in warfare under his belt. Just to become an expert in
only engineering, structures, ditches...
My humble opinion so far of the Sapper as an adventuring profession, is that siege engineer knowledge, knowledge of structures, alchemy, trap setting, rock formation expertise, etc, would all make very good secondary skills for characters.
I'm reminded of the dwarves in Lord of the Rings, and in Dragonlance. They are warriors, clerics, or even thieves. But their racial knowledge of structures, craftsmanship, etc brings advantage to them in the course of their quests and adventures.