Jane was the rightful Queen because Edward VI created the ‘devise for the succession’ which made her his legal heir.
This superseded the Third Succession Act of Henry VIII which returned Mary and Elizabeth to the succession. The third succession act set aside normal rules of succession to return Mary and Elizabeth to the succession even though they remained officially illegitimate and therefore should not normally have been entitled to succeed (neither of them were ever officially made legitimate by him).
If you accept that Henry’s 3rd succession act returning Mary and Elizabeth to the succession was legitimate, then you also have to accept that Edward VI was also legally entitled to name his successor - and he named Jane.
Aside from that, Jane was legally the person who was normally entitled to be next in line to the throne. Mary and Elizabeth were both legally illegitimate so had no normal right to be considered in the line of succession. As Edward had no legitimate siblings, the line of succession went back to the siblings of Henry VIII. His elder sister Margaret had been struck out of the succession when she married the King of Scotland and given up her succession rights, and also been struck out of succession by both Henry VIII and Edward VI. So then succession went by the line of his other sister, Princess Mary, who was by this time dead. Her daughter, Frances Grey, was actually entitled to be Queen, but gave up her rights in favour of her daughter Jane (which she was entitled to do).
People might feel that morally Mary I was entitled to be Queen, but at the time, legally, she had absolutely no legitimate claim at all. She was an usurper. Jane Grey was the rightful Queen.
Edward named her as his heir, superseding any rights Henry VIII had given Mary. And even if you ignore the right of Kings to name their successors; Jane was still the rightful Queen because she was the next in line due to succession not recognising illegitimate children - which Mary and Elizabeth legally were.
It’s absolutely impossible to argue that Jane wasn’t the rightful Queen - she was the legal and rightful Queen by any formula.
People tend to assume that she wasn’t the rightful Queen because Mary I successfully deposed her. But Jane was the rightful Queen.