This is all very odd.
In most cases, you don't get the chance at an Employment Tribunal to demonstrate whether a dismissal was unfair until you have 12 months continuous service (now being extended to 24 months). However if the reason for dismissal was 'automatically unfair', the 12 months service issue doesn't apply.
I'm surmising that your lawyer wrote to the employer on your behalf to state that you considered the dismissal reason as for an automatically unfair reason. That would have triggered you to have your day at the ET (even without the min. of 12 months service), and where the dismissal PROCESS followed would have been demonstrated as fatally-flawed (failure to follow the ACAS Code of Conduct, from what you said about what happened, because you should have been offered the right of appeal of the decision at the outset, amongst other procedural errors that seem to have happened).
Either that, or boss D get hold of it and said something akin to 'this is ridiculous, we can't have staff dismissed in this manner' so he allowed the appeal.
I would take the job back - jobs aren't the easiest to find right now, even for the most able of people. Besides, the compensation payable under the ET guidelines is feeble - its the same as the statutory redundancy package which means you'd perhaps get £1500 tax-free. There's no obligation for the company to pay as little as the statutory sum (they could pay more) but you've only been there 8 months, so it's never going to amount to much.