Hi, humping expert here.
Shoulder humping IS possible, but it's gonna be really hard to feel good. Considering that humping should be executed on a narrow path to achieve optimal pleasure, an average shoulder would be too wide to do such.
Now, Kaho could indeed use Renako's clavicle to hump, which is more plausible in this case, but Renako would then be at risk of having her clavicle in pain (considering the clavicles are quite fragile (Anecdote: I used to have my ex girlfriend bite on my clavicle, and it was really painful)).
Thus, I conclude that shoulder humping is an enticing thought in theory, but it'd be hard to execute in real life.
And idk about armpit tribbing. How and why would anyone feel good about that? Both parties' arms would be exhausted and cramped by the end of the session.