If Doucoure and Allan aren't fit - which I agree is a possibility - you ultimately still have a week to prepare. If that means a week of drilling Gbamin and Onyango then so be it mate. Yes the confidence wouldn't be as much but you're still going with that system, that plan of attack so to speak, but with poorer players, granted. It's about numerical superiority first and foremost. Get a platform, build from there. 100%
You are also right about being defensive. Now it's why I'd play an unknown quantity up top. If that's Simms, so be it, give Brentford something to worry about because we've seen Rondon can't do it. However, if it has to be Rondon, have the plan that he either comes off or is helped out by bringing Simms on on the hour.