You obviously read too many crime thrillers. You aren't thinking this through logically at all. I'm a member of Mensa, score extremely highly on logical reasoning. You're just incorrect and not wanting to back down to save face.
There's hardly anyone living off grid in this country. The examples people keep giving are of homeless people and how thats possible but thats just not the same at all. People sleeping rough aren't being looked for by the rest of society, they just quietly go about their business in the background. They tend to live in major cities where they can beg for money, receive donations, access homeless shelters, maybe steal food from shops or even rob people or sell sexual favours. The bottom line is they need food, and to get food you need money or third party help, or to take it from somewhere. The reason these people can go missing (and why serial killers often target prostitutes) is because nobody is looking for them. People also don't choose to live like that, its through necessity when their life falls apart for some reason - she's got a home, a family and a decent job and its just totally infeasible.
Nicola is now well publicised and if she walked down a street, probably every other person would instantly recognise her right now. How would she get food? She'd never be able to work again, sign on for benefits, and if she slept rough in a city and lived as a homeless person she'd be spotted sooner or later. If there's third party involvement to help her do it and she's staying with somebody, its not just going to be an acquaintance is it, it would have to be someone she knew very well and who wouldn't say anything, close family or friends. And she's not going to be able to live another 40 years like that, the truth will out eventually.
Likewise the suicide theories. I get you can decide you've had enough at any point and we don't know her or her state of mind, but that doesn't mean you don't have to plan what you'd do to end it. You have to have a surefire way of doing it. Thats why people jump in front of trains, hang themselves, cut their wrists, jump off cliffs, take overdoses... its because it has a very high chance of working if you're not found or stopped before you do it. You don't go for a walk and then suddenly think 'oh my life sucks, think I should kill myself, hmmm how should I do it guess I'll just put my phone down here and jump in this shallow river and hope I manage to drown' and on top of that it just randomly happens that her body miraculously floats away much further defying experts' logic. I mean come on, just think things through.
I thought all of this stuff is so obvious but some people seem to think its a valid theory, to me thats just dense.