... Aikido is different and should be to most other martial arts and long may it be so. No comparison. Different purpose and potentially therefore a different level. ...
aikidō is just one of the Japanese budō. No more and no less.
It is the one budō I chose to dedicate my life to. But that makes it different only for me.
In itself aikidō is not different but is comparable to every other budō. Be it technically, be it on a philosophical or spiritual base. And not only Japanese budō, aikidō is also comparable to methods like nei gong or philosophies like Daoism.
There is a countless number of connections, aikidō is not unique and it is not isolated from the rest of the martial world.
So if you want to use the sword correctly you have to look at school which use the sword, to better understand your own skills and methods.
If you want to apply atemi, you have to campare your own knowledge to karate, maybe boxing, to better understand your own art.
Same with kicking: You named a shihan. Christian Tissier - who btw. practiced kickboxing himself - practiced with karateka to have someone kicking who knows how it really works.
If you want to learn how to strike, you have to compare your knowledge with someone of a striking art.
When it comes to grappling and pinning, it is usefull to practice with judoka for a better understanding of one's own practice.
If you want to learn about breathing or guiding your feeling/ki or about aligning you body, you may look into methods like nei gong or tai chi or even shiatsu to better understand your own background.
In my eyes this is crucial. It doesn't mean that you learn pinning from judo, striking and kicking from karate, and so on. You learn and practice aikidō.
But if you don't relate it to the arts it is related to by it's nature it will simply loose it's real skills (which means you / I will never learn them) and will become something only existing in the minds of its practioners as a phantasma.
Doing my job among other things as a teacher, I know that what is an 'A' for one teacher may only be an 'F' according to the criteria of another. I don't share your criteria. I seem to do something different from what you think aikidō is. Allthough I also call it aikidō.