Nothing wrong with her diction IMHO. Yes, she has an accent. I still understood her, despite my hearing difficulties.
My theory is (as for the UK Post Offices!) "use or lose it": if you want to be able to sing in a range of an octave or more without jumping octaves, practice singing scales and arpeggios starting from low and progressing upwards until you really can go no further. Sing along with people who naturally sing higher than you (only if invited in choruses!) I used to sing in various am-dram groups, where you had to sing in the key that was written, rather than choosing a key to suit yourself, (as many folk singers do) - very good for keeping your range as wide as you can make it!
Also choose songs with a big range to keep up that wider span of notes: no cheating by jumping octaves, as my Dad always used to do in church!