Why worry about the way Tess pronounces words, or the reason why she sometimes appears to change accents mid-sentence. It's really a non-issue.
If anyone still remembers the late John Thaw, even he, as an accomplished actor rarely managed a complete episode of anything he appeared in without several changes of accent. No one bothered about that, his acting was the main attraction.
With Tess her presentation and management is the main thing - not small changes in accent and so on.
Probably most people have inconsistent accents. I know that when I am "at home" my accent is markedly different to that used when I am addressing a group of strangers on some technical or educational issues.
Indeed, in family circles I may well say, for example, something like "'Ow are yer terday?", whereas in a different environment I may well say "How are you today?"