Former elder here.
As long as you are not observed (by two witnesses) to spending the night with her, you should be able to avoid DFing. You will have to "really" not be intimate (f*cking) her, or you will have to be really discreet and not be caught.
The secondary risk is "dating an unbeliever". In the past this could be considered "loose conduct". I would fear that it might be construed as "Brazen Conduct" if you are counseled about it but blatanty (brazenly) ignore that counsel. This new "brazen conduct" is a "blanket cover all" for anything the elders want to use it for. If they are "after you", they could twist it to use against you. If they "want" to turn a blind eye to your relationship, they can do so.
Actually marrying the gal will get them off of your ass.
Good luck!
The greatest revenge is living a happy and successful life!