The Protestant Scholastics hashed through this difficulty in a way that I find satisifying. I am saying that the covenant and election overlap but not perfectly. You can be in the covenant but not elect. We are agreed about this.
But this is exactly the what Protestant Orthodoxy (Olevianus and Turretin, for instance) meant when it distinguished the essence from the administration of the covenant.

The beauty of Scholasticism was the ability to take into account the “whole counsel of God” and thus be able to hedge our bets and make proper distinctions.

So, if you are part of the administration of the covenant you have a true union with Christ’s ecclesial body (administration) but not His mystical body (essence).

Would you agree with that?

It seems to me that this search is actually motivated by unbelief, since it seems to be looking for a ground of assurance deeper than the promises of God in the gospel. God has addressed His promises to me as publicly as can be – and I don’t need anything more. I just need to trust Him.

What “we” are saying is that in Ephesians 1 and most other places, “election” is God’s placing us into His body by conversion or baptism, whichever comes first so to speak. THAT is an election I can take to the bank, for I can look straight at God’s promises and trust them. There needs to be a good deal of care taken over the use of this word “election,” and I think that has been part of the problem of late.

I will let the Doctors of the Church give instruction.

1) Yes.

2) Here’s the nub of the discussion, I think. If “saving efficacy” means “efficacious to eternal life,” then the answer is No. Not everyone who is baptized into union with Christ in His body is eternally saved. If you’re asking whether baptized reprobates (or at least some of them) participate (for a time, in some way) in the benefits of Christ’s saving work in some lesser way, the answer is Yes. To go no further: The church itself is a product of Jesus’ death and resurrection, and so membership in the church itself involves participation in a benefit of Christ’s saving work.

To turn the question back to you: Does “legal and covenantal” include some sort of personal union with Christ? Or is it a “merely external” link? Are you meaning to contrast “legal” to “organic” or something similar?

1) Do you recognize a that covenant and election to perseverance (to invoke Augustine) are not completely overlapping? (That is to say that some are elect to eternal life and in the covenant and some are destined to fall away but in the covenant.)

2) Would you say that your formula of sacramental union with Christ is legal and covenantal but not necessarily partaking of saving efficacy?

For instance, our congregation would never rebaptize a former Roman Catholic or anyone else from a Trinitarian background.