These will take a significant bit of editing, but I feel it's worth it.
1. Looking at 5E and other recent game releases, the rules are addressed to 'you' and 'your target' not 'attacker' and 'defender'. You use 'attacker' and 'defender' in places where no damage occurs and no one chooses the Attack action, which can be confusing. (ver 1.23 page 32-33, Grapple, Knockdown for example)
2. In some places, you have phrasing like, "the character must make an opposed Quickness check" (ver1.23, page 30, Exposed) which is not really confusing but technically incorrect. Characters don't roll, players do. This is more to help keep you, the author, in the headspace of "what are the people sitting at the table supposed to do?" and not get caught up immersing in the fiction when you're writing the rules. Save immersion for the sidebars and short stories.
3. You need to use 'round' and 'turn' correctly, in exactly the way 5E uses these terms. This will make it easier for 5E players and avoids confusion and timing issues. A 'round' is 5 seconds of game time, in which each PC and NPC gets one 'turn', taken in initiative order. You made an intentional decision NOT to copy 5E's Reaction rules, so you need to be very, very careful with your timing. Players and the GM can use actions at any point in a round whether or not it is the character's turn. Restrictions apply and need to be made abundantly clear. Your references to the Delay actions sometimes being 'reactionary' are not very clear. You should probably just avoid that word and describe the practical effect you want in each case.
This isn't just terminology but there are many places in the rules where you explicitly lay out a rule, then make application of the rule at the referee's discretion. There may be circumstances where this needs to happen, but be aware of how it reads. Whenever you do this you are undermining your rules and the player's power and authority.
"When you (player) do X, then do Y. Actually, no, just do whatever the GM wants."
Have some faith in the players that will use these rules, let them engage with the game and have their say in the conversation. When you do need the GM to make a judgement call, don't leave him swinging in the wind. Give concrete examples of how to make those calls.
ver 1.23, page 31, Move -- "The referee may allow combined Move Actions"
ver 1.23, page 32, Delay (Standard) -- "An action delayed ... may be utilized at the referee’s discretion as the character sees fit."
ver 1.23, page 18, Assisted Check -- "If the referee determines that the situation would allow for it, one character may assist another." Maybe unavoidable GM judgement call here, but provide guidance and examples. Or instead of "if the referee determines" you could say "if you are close enough to a character and could perform the action yourself, you may assist ..." Use things established in the fiction as the guideline.
