This appears to me to be either a design flaw or else is a programming bug in the RM user interface.
The root cause is that the Unknown Spouse is not really an unknown spouse, per se. Rather, it’s a null spouse, which is to say there is no spouse there. The Switch To mechanism that you are using allows you to Edit an existing person but not to Add a new person. In your case, there is no existing person and you need to Add a new person. In other words, the Add operation is not supported by the Switch To tool.
I think this needs to be fixed in RM by doing one of two things: #1, don’t offer the Unknown Spouse as someone to whom you can switch, or #2, offer you the opportunity to switch to the Unknown Spouse by adding a new person to the RM database for you with no data yet filled in. That way you could just fill the data in and all would be well.
I don’t know if either one of these fixes will be added to RM. For your purposes, option #2 would clearly be preferable. Otherwise, you would continue to need to close the Edit Person screen before you could add the Mother. However, I think option #2 would be very difficult to accomplish given a current design feature of RM.
Namely, there is a major distinction between RM’s Add Person screen and RM’s Edit Person screen. The Add Person screen is the only way to type a new person into your RM database. But it only supports names and sex and facts. It doesn’t support notes and citations and media files and things of that sort. I have long wished that the Add Person screen was more like a blank Edit Person screen than like the Add Person screen it is. For that reason, I tend to enter the barest minimum of information into RM’s Add Person screen and to get to RM’s Edit Person screen as quickly as possible.
But upon further reflection, maybe there is a way that a using the Switch To tool for an Unknown Spouse could be made to work. Namely, the existing Add Spouse tool goes to the Add Person screen and then transitions automatically to the Edit Person screen. If the Switch To tool could be made to do that for an Unknown Spouse, that would solve your problem. For that matter, maybe the Add Person screen could always just transition to the Edit Person screen. Well, some users wouldn’t like that feature. But for me as a sample size of one, it would serve me very well.
This message is already way too long, but let me finish with the following observation. A Switch To that goes to the Unknown Spouse almost seems to do what I wish for. Which is to say that it seems to go to a blank Edit Person screen on the left side of the screen. But none of the fields on the left side of the screen are editable because a new person has not actually been added. But the right side of the screen still shows the data from the original person before you do the Switch To. That’s a real mess and it need to be fixed. The more I think about it, I think that a Switch To that goes to an Unknown Spouse should simply do an Add Spouse and that would solve all the problems associated with this issue.