If I have ships that are twice as fast as the enemy's and I want to stay at long range while he wants to close to short will he still have one of three rounds at short range? In theory you should be able to keep the range you want in such cases!

Speed refers to supralight movement between nodes. During engagement all ship move at constant speed so to be able to aim.

That's how gameplay works, speed isn't a factor during combat so that not to overcomplicate things and not give long range unfair advantage. But some engines contribute to ship evasion stat.