so my friend and i tested if you can enter ribbon syndicate with a pokemon from trade and found something weird
I gave my monty first, tried with 10 ribbons (excluding effort ribbon), didn't grant entry, and when we came back with the effort ribbon (so now 11 ribbons), still no entry.
I got entry way back tho, when my Dewgong only has Effort ribbon and 10 ribbons from the pokemon contests, no e4 champion, gorgeous and royal ribbons so 11 ribbons but i still got in. I kinda assume that the game checks for contest ribbons instead.
another thing is that bulbapedia said that you need 10 unique ribbons spread across your party, but i don't have another pokemon with less than 10 contest ribbons. Hoenn contests are busted rn so i can't get more ribbons. One thing i tried to check was to trade monty and a mon that has battle frontier ribbons (2 extra ribbons) to have access, but my friend wasn't granted entry.
when contests are fixed i'll try to check again if you can spread the ribbons across your party