Rudeofus, trying to summarise: Crucial to success or failure is the time interval between dissolving Part B and then adding Part A. If you do this straight away then no harm done?. If you leave the dissolving of part A and adding it to Part B for more than a day then the Ascorbate will have deteriorated but to an unknown extent thus the risk is there but it is impossible to say at what point along this "delay curve" that mixing Part B with Part A becomes hopeless. As a pointer and assuming the OP made the mistake only a few hours before he posted and has done nothing yet, then the mistake is roughly 36-48 hrs old now
That's one of the difficult things: we just don't know how quickly Ascorbate deteriorates in a given aqueous solution. Decay rate depends on temperature and, of course, availability of Oxygen, but it is also a reaction catalyzed by iron and copper ions. These can come in through tap water, allegedly deionized water, or as chemical impurity from other ingredients. Ryuji Suzuki posted, that decay can be substantial in quite short time frames, less than an hour!
Turning now to your second paragraph which looks like a method of restoration, can I ask if this restoration can only ever restore the Xtol to paper strength developer but restoration to film strength developer is not possible? If this is the case then can you say why restoration to film strength is not possible? Secondly I am a little unclear whether the life of the Xtol becomes permanently adversely affected if there is more than a very short interval between mixing B then adding A, say a few hours only? I am always looking to expand my knowledge
Our expectations for film developer are much much higher than for paper developer, both in terms of reliability and in terms of consistency. Modern multigrade paper has builtin contrast, and it takes more than being a bit off with the developer to really change tonality. If developer pH goes down, you need longer to develop your prints, but who cares? You see development while it happens, and the final result will be the same. If the developer is completely shot and the print stays white or creates funny patterns, you mix a fresh batch, toss that one sheet which said "developer is toast!" and redo the print, this costs you less than US$ 10 and 10 minutes of your life. With film you lose the whole roll, and there is nothing to bring it back except for a reshoot.
That's likely the reasoning behind Darko's suggestion.
Finally I take it that a film leader development test will still be a good indicator of whether B after A has worked but depending on the answer to how this affects longevity it would be sensible to do such a test each time?
When the "sudden death of Xtol" craze boiled over, some people claimed they did a clip test to their satisfaction, yet the actual negs were completely blank. I am certainly not the one to judge or verify the credibility of these claims, but a roll of Delta 3200 is more expensive than the Xtol one tries to save here.