As promised, to analyze some of the above questions raised... (apologies for the long post)...
1. Sociological research shows that regional stereotypes and accents strongly prejudice our perception of someone, either positively or negatively.
At the beginning, Jonny's club-18-30 rep behaviour reinforced the "cheeky chirpy working-class Geordie bloke" stereotype, of which peeing in the shower is considered acceptable and even 'cool', and anyone who dares say otherwise (ie. Alex) is condemned and ostracized as a "hygiene-freak". (To take an extreme contrast, the public reaction to the Queen saying she pees in the shower would probably be different! A less extreme but more telling comparison is to ask: What would the public reaction be to an exposé of urine-stained showers on a hospital ward? - I can see furious news headlines alluding to 'filthy hospitals'.)
LABELLING (typecasting) is extremely powerful in colouring our subsequent perceptions. Hence despite Jonny being morose and glum on the poor side for weeks, the general popular perception of the "funny Geordie" remains largely untarnished; contrast Sophie or Sandy being glum were both condemned by the public as being "boring". (Similarly, Alex labelled "the moaning model" result in people looking to construe anything he says as whinging, no matter how innocuous the remark -- eg. DS reporter Emma Tingay in a live update: Alex commenting on the weather becomes " 'It is raining,' moans Alex." (Would the same verb have been used if another HM had said the same? I doubt it.)
Does Jonny tries to cash in on his Northerner background as part of a national North-South rivalry?
Undeniably yes. Jonny, in conversation, said that he "represents the North, Scotland, and Ireland"-- by implication, he is appealing to the inhabitants of those areas to support him, by attempting to tap into tribal loyalties. In addition, his impersonations of Alex and Tim's accents panders to prejudice towards both regional and class differences, because - importantly -he does them with derogatory overtones: eg. "Golly gosh, posh people speak like this". (Contrast with the one time Alex imitated Jonny's accent in the diary room when he used it purely to assume Jonny's identity in relaying the conversation between them; the difference here is that Alex did not do it for derogatory effect.)
2. How significant is his fireman status as a factor in his popular appeal?
It helps - and certainly doesn't do him any harm. The general public have a (rightly) healthy respect for emergency service workers, and of these, traditionally firemen have a somewhat romanticized image (even more so after the New York attacks). This makes people naturally well pre-disposed to them. Some Jonny supporters seem to base this positive prejudice as a reason for their support, eg. comments along the lines of "Jonny does a more worthy job" "the world needs more firemen, not models".
Does he play on this?
No, that would be an unfair claim. All the other HMs have talked about their careers, usually in response to questions from other HMs; people are naturally interested in others' jobs, especially interesting ones like firefighting or modelling.
3. Does he contrive his behaviour to appeal to public popularity?
This issue has been extensively covered on the forum already. Read Elysium's threads! Interesting also to note that, in her analysis on last night's Sunday show, BB psychiatrist Sandra Scott described Jonny's "court-jester" behaviour as "artificial".
Does Jonny attempt to 'hijack' camera attention away from others?
Yes he does. Dr Scott got there before me on this one! She highlighted eg. Jonny's frequent "Big Brother" narration during other HM's activities as a method of drawing attention away from them onto himself.
Final point. Why has Jonny managed to successfully create the illusion that, as Jonny-supporters often assert, that "he does not bitch about the others?" One important reason for this is his clever use of language.
Compare and contrast —
1)Adele to Jade: "What do you think of Kate?"…(agrees)…"very dodgy"; "PJ/Jonny is a *****r".
2)Jonny to group: "Who do you trust in here?"… "I only trust you (Kate) and PJ"; "Is there anyone here who gets on your nerves?"
--Their different ways of effectively saying the same thing have different subconscious effects on the observer ie. the viewer.
Adele's use of person-specific ("Kate" "PJ" "Jonny") explicit direct attacks on someone ("dodgy", "*****r") has the effect of focusing observer antagonism towards her own character for obvious "bitching".
Jonny is much more subtle in his "bitching" about HMs he dislike. By phrasing the question in non-person-specific positive language - "WHO do you TRUST?", and stating the positive to imply the negative ("who do you trust?" = who DON'T you trust?; "I only trust [Kate] and PJ" = I don't trust Alex/Tim/Jade), he thus neutralizes viewer antagonism and create the illusion of not bitching. Another important feature (going back to my comments about the powerful effect of labelling) is -- GET YOUR LABEL IN FIRST!!: By referring to himself, Kate and PJ as people who can be trusted, the subconscious effect on the viewer is to identify them as "trustworthy" and, by implication, paints Alex/Tim/Jade as the untrustworthy people.
As the pop lyric (almost!) goes: It's not what you say - its the way that you say it. Politicians, lawyers (think George Carman) - and trained actors (Jonny?) - are very aware of the subconscious power of judicious language use on an audience. It was interesting to note how, in last nights show, when Jonny was in the diary room he was clearly slow and deliberate in searching for the right words in saying (and getting his label in first!) - "Me and Kate are here to have fun... Tim and Alex aren't."