He used the present perfect because the original sentence used present progressive. If the original had been "The market expands considerably" then "There is a considerable expansion in the market" would match. However, the present progressive indicates an ongoing process that your new sentence should reflect. We can't say "There is being a considerable expansion in the market" because that would just sound silly. The present perfect is better because it shows that the expansion is not just a present condition; it started sometime in the past and continued to the present.
1) You do not put "a" before "consumer confidence" because the noun here is confidence, and consumer is acting as an adjective. If you want to treat consumer as a noun, you could say "a consumer's confidence" or "the consumers' confidence". Confidence (the noun) is not singular. 2) You do not put "a" before "consumer confidence" because "confidence" is not being used as a singular noun. It is not one consumer confidence, but an indeterminate amount of consumer confidence. I do not put "the" before "consumer confidence" because I am talking about something general rather than specific. I can talk about "green apples" in general or I could talk about "the green apples" in my bowl, though the latter would actually depend on having green apples in my bowl where, sadly, I have none.