User blog comment:B214/Question and Answers/@comment-95.234.163.134-20131213181733/@comment-1359322-20131214015302

1. Sacred Gear are connected to the body of the possessor. Like how Issei can be affected by Dragon Slayers because of him having the Boosted Gear, so Yuuto also has the requirement needed to wield a Holy Sword after Volume 3. (That's why he has the Blade Blacksmith in him too)

2. It's only said that they did some modifications on the sword so that Issei can assimilate it into his Boosted Gear, well Azazel helped in the process and him knowing a lot in Sacred Gears probably made it possible.

3. Yes he can channel Ascalon's power to his Gauntlet but he can't give it to others Dragons, Ascalon is modified specifically for Issei to use to fend off other Dragons, used it on his ally Dragons, and they'll receive damage instead.

4. I believe you can find it in Xhatefate's blog.