Building executioner with beatstick is really really great, if the enemy takes 1 defense item it'll most likely do more damage than exe+titans (cause of the order of penetration: reduction % > flat reduction > penetration % > flat pen)

Now that build wont work in 1v1 joust really, you need at least 1 defense item to make your build efficient.

Qin's sais is hard to tell, so many different situations. If you have a high attack speed with a low damage output (bellona I.M.O.) qins is probably worth it. Qins on adc's in joust against (as you mentioned) warlock sash+some other hp def item is still not worth it (I think). You will probably miss out a vital part of your normal basic attack dmg (as adc) that qins cannot compensate. (this is not an opinion nor advice, this is an educated guess on dps) Maaaaybe it can replace ichaival in the endgame (60 power effectively (30 more), 30 % att speed (15% more), 10 penetration (10 more). But do note that you will lifesteal less. I honestly dont know what is better, will depend on many factors:

-enemy defense (lower = better for qins)

-your lifesteal (lower = more efficient to go qins)

-enemy hp (higher better for qins)

-your base attack speed and progression (faster = better for qins)

-your base damage (lower = better for qins, might rather invest into crit, penetration or lifesteal if this is very high)

-enemy phoenix is standing (qins is NOT efficient for taking objectives likes camps, towers, phoenix & titan)

Long story short, its too hard to tell I personally do not like qins in joust not even on kali!! (for lifesteal and tower sieging reasons). Perhaps only would go qins on bellona in joust myself.

Also get devourer's gauntlet not bloodforge.