It's quite normal for CR arguments to require multiple assumptions. In fact, we could often continue to brainstorm lots of smaller assumptions all day long. (E.g. "The money in banks won't be embezzled and invested out of the country," "Aliens won't destroy Earth tomorrow," etc.) It's often hard to say that one particular assumption is the "crux." Harder questions will sometimes focus on more obscure / harder-to-predict assumptions, but we rarely need to consider them all. We just need to find an answer that addresses one of them.
And yes, the assumptions you identified are required by the argument.
Gaps or assumptions that I noticed, basis the wording of the passage, while trying to answer this question --
1.
'induce' -- Govt will try to induce, but at least some people will actually start investing in retirement savings accounts after tax changes.
2.
'individual taxpayers' -- Money deposited would be significant compared to dev loans requirements.
3.
'available to borrowers' -- At least some of the borrowers will be 'businesses requiring development loans'
The OA is based on the last one and I got the answer correct.
However, I want to understand the correct thought process/framework for such questions with seemingly multiple gaps -
1. Are the 1st 2 assumptions valid?
2. Even if these are valid, does GMAT usually make questions based on such assumptions? Or are they quite far-fetched and GMAT would usually question only the crux? (Eg here the crux is -- borrowers != businesses requiring dev loans).