To convict you of the offence of speeding, the opinion of a single witness is not sufficient, it has to be corroborated - for example by performing a follow check. Measuring your speed through a follow check, or measuring his own speed whilst you are pulling away from him (meaning that your speed must have been greater than his) can suffice.
However, you have not been charged with speeding - either because there is a question mark over whether the evidence is sufficient for that offence, or because careless is a more appropriate charge - so the above appears to be about as relevant as the advice given by some of the professionals you spoke to. As a general rule of thumb, short of dangerous driving (and even in some cases where dangerous might be the more appropriate charge), the police will go for speeding if the evidence supports it because it is easiest to prove.
As an aggravating factor for careless driving, the only question regarding evidence of speed (ignoring questions of overarching admissibility under s. 20 RTA 1988 if a device of a type prescribed is not approved) is weight - having potentially heard evidence from both sides, which version do they prefer.
Do you have evidence that there were potholes there, other than your own testimony? When you were cautioned at the side of the road, and told that failure to mention anything that you subsequently rely on may harm your defence, did you explain that you were swerving to avoid the potholes? Were the other cars also swerving to avoid the potholes, or were they just ploughing through them?
If you wish to admit the offence, but dispute the alleged facts you can enter what is known as a Newton plea. The court would then have to accept your version, or hold a Newton hearing - essentially the same as a trial, but solely to establish the facts that are material to the seriousness of the offence, and the sentence.
If you do not object to a witness statement, you cannot subsequently challenge its veracity, although you can still comment on it and point out any self-evident flaws. I would say that the court has no right to read more into the statement than what it actually says, but I would be wary of relying on them not reading between the lines. That said, if this goes to trial, I would expect the occifer to attend to give evidence in person - at which point he is gioving his evidence, not the witness statement. Tactically, objecting to the statement would mean that there was no evidence if the occifer failed to turn up (unless the case was adjourned to enable him to do so).
The dashcam evidence is unlikely to be needed to convict you, but might help your case if the occifer's recollection was clouded.