Dainty,
Tea serving after ROM sounds weird...have not heard of anyone doing this so far, usually it's done on your AD after fetching of bride in the morning. Unless you are holding your ROM and AD on the same day then that's a different story.
If you are still planning for your AD on Sep this year, you do not have much time left...bear in mind that IF your WG is MTM, it will take 3 months or more to make, and after that you still have to go down for trial fittings and make alterations from there. Also, PS is usually done about 3 months before your AD, so you can see you really do not have much time left since you have not even settled on your BS and venue. Do take note that most of the hotels now are already fully booked for weekends for this year, so you may have problems trying to find one unless you don't mind having it on a weekday. If you are still planning to have it in Sep this year, do confirm on your BS and venue quickly as you have no more time to lose.
How bad is her condition now? Is it terminal illness or? I think you need to sit down and have a heart-to-heart talk with your FH whether it's sensitive or not as this matter concerns the BOTH of you. Let him know what are the possible consequences should his mum pass away suddenly. Time is not on your side too, so you really need to give this matter serious thoughts. It is still ok to hold your banquet as per normal if it's within 100 days after her passing, no need to scale/tone down or anything. I have friends who proceeded with theirs within 100 days too. Wedding banquet is part of customary wedding.
For your info, my wife and I are holding our AD in March next year, her father passed away in December last year. Have checked with our master and he said it's ok to hold our banquet next year. Both of us are in our 30s too. Probably you can go consult a master too and ask for his opinion? Let him know of your situation now, he can advise you on this.