Well, where I come from, the rules for cover drivers went like this: when the vacation list went up, it was picked by seniority, first bid drivers, then unassigned. All picked by seniority. If a cover driver didn't want to cover a vacation route, he picked "float". The list came down on Friday, it was finalized, and posted by Friday afternoon with all of the picks. Come Monday, when call-ins, optional days and base cars that didn't have regular drivers opened up, the picks, again, went by seniority, first bid drivers, then unassigned drivers that didn't pick a vacation. Drivers off on Comp or disability had their routes held down, again by seniority.
Now, managers always tried working the bids to their advantage, but that stopped when drivers simply stepped up and said, "NO, this is the route I'm going to run." They always tried getting newer drivers on the easier routes to make things run smoother. Normally, this was because they would never run a route with a newer driver, fearing that the route wouldn't get done. So, they tried twisting arms to get more experienced drivers to run routes they might not want to run, if only because they had ran them before. But that is not the driver's problem. But, and it is a big but, the driver had to stand up for him/herself and demand to run the route they wanted (as long as was their pick to make). There is nothing managers can do if a driver has the seniority and demands to run a route in their right to run.
When I was a bid cover driver (waaay back when) I had many heated arguments with sups because I knew every route in the center. So, some days, there might be a real nasty, ball-buster that only I knew how to run. But I was second on the list to pick, and normally I would pick a route I would WANT to run. I always used to warn sups that they needed to train newer drivers on as many routes as possible because when they wanted me to run a route I didn't want, they would be friend$cked. Yeah, yeah, we will was what they always said. Did they train though? Of course not.
So, before the message was cemented in their heads, they would try all kinds of tactics to get me to run some POS so they wouldn't have to worry about getting that route done that day. It puts a smile on my face thinking back on some of those blow out arguments on the boxline and in front of all of the other drivers. Man, they used to get fired up, usually because they realized one of them was going to have to brown up and run a packed P-1000 in some industrial area with a new hire. But you know what? Tough testies. My job wasn't to fill management holes and to make a sups job easier. My seniority and it's benefits didn't stop because they didn't plan accordingly. What is the expression? "Lack of planning on your part doesn't constitute an emergency on my part." They always hated hearing that one.
You OP isn't real clear. It lacks some details. And different areas have different agreements, but it sounds like this sup is protecting your female driver. Maybe because he knows she will go splat on routes she doesn't know. Maybe he is banging her and hates you. You're not the first. But if you have the right to pick over her, well, you gotta put one foot in front of the other and stand firm. Don't get mad, don't start yelling, and for your own sake, stay off the DIAD messages with personal opinions. Tell them, "No, this is the route I will be running today/this week/or for the duration." As long as it is in your right to do so, do so. Yeah, they will get mad, maybe even threaten you. All of that doesn't matter. You can file later on that, but run the routes you are entitled to run. Stand up for yourself, and be consistent. Trust me, if you show them that you are sticking to the rules EVERYDAY, they will eventually try this crap on drivers further down the seniority list.
Just pay it forward to those drivers when you see them getting the scroogie.