Speaking strictly in a technical term, Yes the crown "could" use what is said during a negotiation in court, When you go in for a first appearance meeting with the crown, there are signs in the room stating that the meeting IS being recorded, and generally the crown, will also advise you that the meeting is being recorded. *** They are recorded NOT to use what you say against you, BUT rather to "protect" the crown, so that you don't get up in open court and say the crown said if you had sex with him/her, they would drop the charges. ***
So, in real world practical terms, NO what you say during the meeting with the crown is NOT used against you. Remember, the whole idea of the meeting is to offer you a "deal" in hopes you will accept it, to prevent a complete clogging of the court system. Therefore, it wouldn't be in the best interest of the crown to begin using statements from the meeting against the defendant in court, as MOST people would simply not take the meeting, and cause the courts to clog up.
Now, to the OP. As someone who has spent countless hours sitting and testifying in traffic courts, "MOST" paralegals, and attorney's, (unless there is a GLARING issue with the ticket), are going to go for the "win" which means they get you a reduction in the charge and/or in the penalty. Something you too can accomplish via the first appearance meeting with the crown.
Again, without seeing the disclosure, no one here, (including me), can tell you if this is a winnable case. Anyone, (including a paralegal, ex coppers, lawyer), who tells you they can "win" your case without seeing the disclosure is simply NOT being honest, and stringing you along to get the cash from your pocket to theirs...lol
Should you decide to go it alone, in a first appearance meeting with the crown, be POLITE, RESPECTFUL, and dress in casual manner, (preferably not jeans and a wife beater). Listen, to what the crown is offering, then "explain: your issue" (the change in speed limit). and say along the lines of I know I screwed up, but I have LEARNED a valuable lesson about watching for speed limit signs more closely, and respecting the limits. If they offer to go from 49 over, (assuming the officer hasn't noted the "actual speed" on the ticket), to 39 over, again be respectful, and restate the change in limits, and that you would be prepared "not to waste the courts time, and enter a plea, if the charge went to 29 over. At that point your simply trying to lower the fine, (although the points would also be dropped, that is of No consequence, unless one is on the verge, of having to go to a meeting with MTO, over accumulated point loss).
Whatever happens be very aware of your speed in the future, and above all GOOD luck in court.
I always wondered - when you meet the crown in an early resolution meeting, do you admit to the charge when plea bargaining? And if you can't reach a mutually agreeable plea and opt for a trial, can they use what you said against you?