Ditto about the lemon juice. Yes wheels can be a total pain. I had an issue a while back when one broke after 6 months. Sorry you've had some wheel issues. The Karlie wheels are quite sturdy and do tend to last better than the plastic ones, but then there's buying plastikote as well. I noticed you're in Ireland and other members have had problems getting hold of stuff there. Zooplus Ireland does sell the Karlie wheel though. 16.99 euros
Wonderland Exercise Wheel: Great Small Pet Accessories at zooplus
Plastikote is usually another £4 to £5 but a little bottle goes a long way. You could maybe keep the Trixie wheel for the future - either another hammy or an emergency wheel.
Personally I wouldn't use it without the front on. Maybe other members know if that's safe or not.