Can't answer all your questions, but the following might help: - Proc model can do just about everything that syslin can (ie it does non-linear models as well as linear models). The main difference is that Syslin has simpler syntax. - I think your panel data model is a fixed effects model. These can be calculated in several ways (other than using proc panel or glm absorb). Eg by actually including a dummy variable for each fixed effect (eg firm) or by transforming the data so they are deviations from within-firm means. You should look at textbook like Wooldridge's "Econometric Analysis of Cross Sectional and Panel data" for an explanation of this (I just googled it, there seems to be a PDF available via google scholar). The first of these methods (maybe both) will work with unbalanced panel data, and then you use proc model or syslin to do the 2sls. - Overidentifying test. I've never done this (finding one valid instrument is hard enough, let alone two). The idea is that it tests whether both instruments give the same result. You might be able to test it by similtaneously estimating two equations (with differrent instruments in each) and then testing for equality of coefficients - but I'm just speculating here. - Wooldridge refers to a method by Arellano to calculate robust standard errors for panel data, but I don't think is available in any canned routine.
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