Imagine if the Marquis has many children, brainwashed, and used them correctly (meaning marriage with many noble families), the Duchy would've been in a more tougher spot. FMC will have no choice but to summon the spirits and/or going back to her land. By forcing FMC to go back, the Marquis can essentially kidnap her on the way, put her through the most horrendous experience one could imagine for a female (or a human in general), and demand for the Duchy to surrender. Of course, the Marquis would need three to five secret safe houses only known by a select few--not including the children, but the Crystal Cave safeguarded by the Lachia Duchy is just that worth of a spending and secrecy. The Marquis should not forget to throw away the necklace/ring that housed the Blue Flame (if she brought one). The rest is improvisation. There's also the problem of FMC being guarded by the Lachia's Secret Service (Shadows?), but the Marquis should expect something like that as well. Asking for help from the Emperor is also a good idea.