This statement is incorrect, I'm afraid.
b) Clearly, If I were to die, my 75% share would be conveyed according to my will, the minor-holder would have no say in the matter, and presumably the title deeds would be altered accordingly irrespective of their wishes, and could presumably not be prevented by their refusal to sign a document.
The two of you hold the property as trustees for yourselves. If you die the other is a surviving trustee and unless there's a trust deed that specifies to the contrary (unlikely), can appoint a new trustee. The trustees have a duty to hold your 75% share for your beneficiaries but the property itself does not necessarily get transferred to the beneficiaries. If they went to court they could no doubt force a sale.
Actually the basic protection given to tenants in common is in practice not that strong on death. If one trustee dies the survivor can appoint anyone as a co-trustee and quickly sell the property and keep the proceeds. That would be unlawful and the deprived beneficiary would have a right of action against the trustees but if the others disappeared and spent the money the beneficiaries could lose out.
You can assign your beneficial interest but as you and the other person will still be trustees the assignee might have to go to court to force a sale.
c) I do have a potential purchaser who is willing to buy my 75% interest (albeit at a discount), either to be named on the title in replacement, or (as a less desirable alternative) as beneficial owner with the existing tenants in common remaining as trustees (me simply as trustee for purchaser but with 0% ownership).
Yes you would need their signature if the buyer is to be registered at the Land Registry as a proprietor (along with the other) of the property.. You cannot pass the legal estate without this signature. You would have to apply to the court to have the other trustee removed.
d) I have been told tentatively that in order to sell my part share outright, I would require the signature of the co-owner. I find this difficult to reconcile with a) the fact that they could not prevent altered co-ownership on my death by refusing to provide a signature, b) my understanding that a tenant in common is free to dispose of their part share as they wish.
As a conveyancing solicitor I believe the information given in the post to be useful (provided it relates to property in England & Wales) but I accept no liability except to fee-paying clients.