Hi Brent,
The bottom line is that boxing is using up part of your weekly time and energy budget and is not contributing in a specific way towards bike performance. Training for two things dilutes your goals. It is clear that boxing is a high priority for you to keep in your schedule. It is not the type of cross-training I would recommend to other athletes.
1) I have not come across any other athlete using boxing as cross-training for mountain bike racing. It does not replace anything in the plan. It does use energy and time, which are finite weekly resources, so you do need to remove something each week in the training plan to leave room for boxing.
2) This is a definite no! Boxing uses more muscle groups (like other sports such as xc skiing and running), driving HR higher and will have higher HR zones. Bike zones are lower than zones for most other sports, especially ones that are foot-sports like running. Swimming and biking are sports where the body is supported by the water or a bike and require lower HR’s to reach threshold power. You need to test on your bike in a sport specific situation. If you are doing most of your bike training indoors, test indoors. If you are doing most of your bike training outdoors, test outdoors.
Coach Lynda