It is the offences dates that matter - including the current offence, not how long you can drag the process out for.
To all intents and purpose you cannot accept the conditional offer of a fixed penalty. The "conditional" part is that the offer is void if you are liable to tot up.
Unless you are able to successfully defend the latest speeding allegation, and you have not posted anything that indicates that you might have a viable defence, the court will be required to disqualify you for [at least] 6 months unless you can persuade them that such a ban would result in exceptional hardship - beyond that which the average driver would expect to suffer - in which case they can reduce the length of the ban, potentially to zero.
Hardship suffered by innocent parties who rely on your ability to drive is likely to carry more weight than hardship that you will be deemed to have brought on yourself. Expect to need to evidence pretty much any ground you are seeking to rely on, and show that there are no viable alternatives - that the picture that you are painting is the hardship that you/they would suffer, not simply the worst case scenario with viable alternatives either not explored or conveniently overlooked.