My question is related to what is "includible compensation" as referred to in the regulations at section 1.457-2(e)(2). Specifically, are tax-deferred amounts taken from a participants gross income for a defined benefit plan included for the includible compensation calculation?

No. I.R.C. § 457(e)(5) and Treas. Reg. § 1.457-2(e)(2) define "includible compensation" for purposes of 457 plans to include only compensation which is includible in the employee's income (for tax purposes). Thus, amounts which are not includible in the participant's income (including amounts contributed to the 457 plan itself, as well as the contributions to defined benefit plans you mention) cannot be included in includible compensation.
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