image editing in RGB and manual separations to CMYK for press

We'd place the manual color space convertor's work back into our ID doc

Also, in case you are not aware, you can get the expected CMYK conversion for profiled RGB colors without actually committing to the conversion. Here I have Proof Colors turned on with my Proof Setup set to Coated FOGRA39. With my Info panel’s eyedroppers set to Proof Color I can get the expected CMYK values for a conversion to FOGRA39 without committing to the conversion:

Can we execute all needed image optimizations in RGB (mostly B&W architectural images)

In almost all cases RGB B&W images will convert to CMYK using all 4 plates. If handled correctly that can get you more dynamic range on press, but you run the risk of color casts. For color work, color correcting in a large RGB space (ie, AdobeRGB, ProPhoto RGB, etc) with View>Proof Colors turned on is the preferred workflow.

There is no guarantee the color will print accurately—how you handle your display matters. All color in the Adobe print apps is converted into your system’s monitor profile for display—profiled RGB to Lab to the monitor RGB profile, or profiled CMYK to Lab to monitor RGB profile. If the system’s monitor profile does not accurately represent your monitor’s properties (gamma, white point etc) your color correction preview will be less accurate. Typically hardware calibrators are used to generate the most accurate monitor profiles.