You raise 5 objections.  

 (1)What the hell are you talking about? Of course we can assume that Achilles is catching the tortoise at constant speed. If your aim is to disprove the statement "Achilles cannot catch the tortoise (no matter his speed)", you are free to assume anything you like about Zeno's speed. Are you stupid?  

 (2)This is not dubious. This is what the whole point of the section is to make clear - that a stipulation that Achilles is catching the tortoise at a constant rate, or some similar stupulation, is necessary for Achilles to catch the tortoise. I guess you could include proofs, or a link to the harmonic series, for example.  

 (3)Covered by (1).  

 (4)Is pretty obvious.  

 (5)This is simply related to the divergence of the harmonic function. All it needs is a link. The divergence of the harmonic function is not "dubious", having first being proved in the 14th century. Do try and keep up.