so, my question is, if eating rabbits in a long term situation, and ensuring you have your greens too, would adding tallow to the pot at least mitigate or even negate the lack of fats issue
Yes, adding tallow would help. However, one or two survival candles woudn't help you a great deal.
I think that part of the problem that other folk might be having in answering you is that this hypothetical situation is so specialised as to be unrealistic.
Lets re-cap.
You are in the UK, not the far north of Canada anyway, and since it is not the far north everything is not covered in snow so you can get green food. However for some reason can't get any carbohydrates from roots or tubers. You are going to be stuck there for several months without rescue or re-supply. There is an almost unlimited number of rabbits which you can catch and while you planned ahead far enough to be washed up with a huge supply of tallow candles and snare wire, you decided to leave the more palatable and nutritionally useful and easy to obtain stuff like lard, butter, marge, vegetable oil and flour behind.
As "what if" situations go it is hard to fathom.
Clearly you have given this a lot of thought, but you have made comments which run counter to what I have read about "rabbit starvation". For instance you say that people starve while appearing and feeling well fed whereas I have read that it is just the opposite, that protein poisoning is characterised by a lack of feeling well fed, despite consuming ever increasing quantities of protein.
Also, that while there are issues with lack of vitamins and minerals, these are no different from those you would experience if you were to eat any other type of food to the exclusion of all others. For instance, you would encounter problems if you ate nothing but salad leaves. This deficiency of unballanced diets is not specifically tied to the consumption of protein.
The reason that eating some fat can help with holding off protein poisoning is that fat contains a lot of calories and you are no longer forcing your body to try to get its whole daily requirement by processing protein.
Have you looked at liver function and the use of ATP? This is the key factor in rabbit starvation, and explains why people die faster than mere starvation would explain.