Flex Queue Teammate Eligibility

I recently submitted a support ticket to try and get a further understanding of how the ranks affect who you can play with as my friend recently hit challenger in flex and we can no longer practice because of it. In short the answer I got was that on the FAQ page these rules apply to both Solo/Duo and Flex Queue.

Bronze Tier: Can only queue with Bronze and Silver Tier players. Silver Tier: Can only queue with Bronze, Silver, and Gold Tier players. Gold Tier: Can only queue with Silver, Gold, and Platinum Tier players. Platinum Tier: Can only queue with Gold and Platinum players, with exceptions listed below. Diamond V: Can only queue with Platinum I, II, III, and Diamond II, III, IV, V Tier players. Diamond IV: Can only queue with Platinum I, II, and Diamond I, II, III, IV, V players. Diamond III: Can only queue with Platinum I, Diamond I, II, III, IV, V, and Master Tier players. Diamond II: Can only queue with Diamond I, II, III, IV, V and Master Tier players. Diamond I: Can only queue with Diamond I, II, III, IV and Master Tier players. Additionally, Diamond I and Master Tier players can only queue solo or with one teammate. Master Tier: Can only queue with Master, Diamond I, II, and III Tier players. Additionally, Diamond I and Master players can only queue solo or with one teammate. Challenger Tier: Can only queue solo.

First off, why would this apply to both systems and what would the point of getting to challenger in flex queue even be if it supposed to replaced ranked teams now higher elo teams can no longer practice together.

Second off, I found that this doesn't always apply as when my friend was ranked diamond 1 he could play with us when were ranked diamond 5 and also playing as a group of 5.

Last off, when hovering over names you cannot invite for my friend who is challenger it says he must invite someone between the ranks of Challenger and diamond 4.

All in all there seems to be a lot of inconsistency on information or lack of regarding to who can play with who at the higher elos of flex queue.