If it was executed as a deed then the fact that it is undated could render it ineffective - many deeds contain a statement that they are delivered when dated, therefore if they are not dated they are not delivered and therefore are not effective. However, you may find that even if the date hasn't been inserted on the front page it will have been entered somewhere less obvious (like at the end of the signatures) so you'd have to check this carefully.
Likewise, if it was executed as a deed your aunt's signature would need to be witnessed in order for it to be effective. The witness doesn't have to countersign the signature at the time of signing, this can be done later, but the witness must actually see the document being signed. So if your aunt is correct in thinking that she never met the witness, that would also be grounds for arguing that the document is invalid (provided it is a deed).
In terms of changes being signed by one party only, this is a difficult area which has been complicated further by a recent case. It would really depend on what the change was and whether it was agreed by all the parties, whether or not they had all signed the amendment. Again, if the document is a deed there are more formalities to comply with and this is more likely to be an issue than it would be if it was a simple contract.
Difficult to answer the interest point without seeing the wording, usually interest would accrue daily and be compounded but the clause would need to specify. This may be covered elsewhere in the document.
I would usually expect a loan to be documented in a simple contract, in which case it wouldn't need to be witnessed, or necessarily dated, and it would be easier to make an amendment which wasn't countersigned. However, the fact that it was registered at the Land Registry implies that is contains some kind of charge which would mean it should be a deed.
If you are genuinely concerned about this you'd need to get a solicitor to look at the document. The Land Registry do usually check that deeds comply with the relevant formalities and reject them if they don't, so the fact that it has been registered could indicate that it's ok. However, registration isn't definitive and the Land Registry do get it wrong so if you are concerned definitely get it checked - explain your concerns and ask them to tell you whether there is anything wrong with the document - the fact that your aunt doesn't seem to know what she signed is a concern but on the other hand I've known people have documents explained to them in detail and then later claim that they didn't understand what they were signing up to.