What you ask for looks morally justified based on things like continuity of employment and the fact that most other employees in this position were granted the original terms. The fact that 'C' still provides these terms strengthens your position even more.

A provided these terms because it was part of B which provided these terms because it was part of C.
The fact that E happened should be irrelevant.

A key thing for me is "the two of us moved into the new parent company and assured that nothing was going to change"

If you were given limited options about signing with E then that contract may be unreasonable.

I personally think you should be *very* assertive about what is owed you - go see an employment solicitor (My wife did and it was immensely useful). I bet the only reason you're getting 'screwed' is because you don't have the weight of numbers that B was able to use in their negotiation with D.

Good luck