Can I use "some leaps of logic"?

I'm preparing GRE and in my argument, I wrote "Although this argument seems to be reasonable, it suffers from some leaps of logic that should be addressed". My instructor said that "some leaps of logic" was not right. I do not know the reason.

Some other questions: Does "If is at least likely that ..." equal to "It is possible that"? What is the meaning of "If is at least likely that"? Does it equal to "If is unlikely that"?

How to use an adverb to join two sentences? For example, I wrote" It is also possible that the city have more than ten million residents; thus even if hundreds of workers are constructing the highway, the influence is insubstantial enough to be ignored." Is it right? Can I use "and therefore" or "and thus"? Is there any other word to substitute "insubstabtial"?

"To reach his final judgment, the author must provide further evidence to exclude all these concerns; otherwise the argument above is absolutely unconvincing."

My instructor advised me to use "," instead of ";", but when I was writing, I thought that "otherwise" is an adverb so that I should use ";" to connect these two sentences. Why is it not right?

"Overall, the argument suffers from several serious flaws that render it unconvincing. The author would be well-advised to address these defects before taking this to those pundits who oppose the proposal and in doing so, would add support to the argument and perhaps help the author make his case."

Are there problems in this paragraph?