well, for my own opinion,
i think you can use english to read and and write arabic , but ofcuz, some of letter my not exist in english,
than, you have to creat some simbols or specially letter, to replace it. now they are useing those simbols for instance : ( ' 3 , 7 ) for those letters ع غ أ ظ etc
above all, there are many different kinds of writeing as you said, some may write "chanran, while some write "shakran"
just becuz, there are no stardard writing by useing english.
becuz, in french or some other languages "ch" pronounce like "sh" but not in english,
and it also depends on the person who write is from which country, if from french or colonial, they would prefer to use "ch" instead of "sh"