Also that spoilers is from the WN so the LN may be different.... No seriously. Please soothe my soul.
There is pretty much no chance the LN won't go this route as their are only minor differences between the two versions. Also remember that even the raw LN only goes up to volume 20 (volume 21 comes next month) and Part 4 will only conclude with volume 21 (after taking 9 volumes). To add to that, as you just mentioned, their engagement only happens in Part 5 (which encompasses 32% of the WN by chapters, for comparison Part 4 in the WN is 27% and Part 1 is a meagre 11%) and only at the end of that. What I'm trying to say is any LN reader saying that Rosemyne-Ferdinand isn't happening is standing on no firm ground. Sorry.