Meanchem, re the books I referred to which indicated that the Lubavicher did indeed want to become rebbe –
larger than Life by Shimon (I think) Deitsch says that his parents would only agree to the shidduch if the Rayatz agreed to nominate him as the next rebbe. They knew that the Rayatz’s daughter could not bear children, so that’s the reason they agreed to the shidduch. I seem to recall the story was related by someone with the name Althoiz, who was very close to the Rayatz, and he may even have been the one who finalized the shidduch. I know that this doesn’t say clearly that the Lubavicher rebbe actually wanted to be the next rebbe, but it certainly implies that he accepted that as part of the plan. After all, unlike other chaddishe rebbes of the times who married at a much younger age, the Lubavicher rebbe was something like 25 when he got engaged, and I’m sure he had some input into the matter.
The other sefer is more explicit. I don’t remember it’s name, and I have only seen it once when it was lent to me for a few weeks close to ten years ago. It was writting – in Hebrew – by S Z Gurary aka Jimmy, who was close and even family with the Rayatz I believe. Sruprisingly, and this is why my friend lent me the sefer, it details that fights that went on behind the scenes after the petirah of the Rayatz.