We had faced the similar problem for years. A sloping road and an old house are recipe for smelly and nasty disaster. For us, the issue was compounded by the fact that our none of our neighbors were facing the problem as all the sewage water would enter the house.
We have used option 1 and 2 of the suggested solution. It is an expensive fix but needs to be done. As for option 3, electrical pumps will not work because in Bangalore, when it rains, there will be a power cut. If manual pumping is installed, someone has to stand and work on it during the rains.1.To raise your internal chamber level with the leverage of gravity flow
2.Like you thought of using a manual shut down valve ,but it needs to be regularly checked as the valve would corrode very soon due to heavy acids in the waste water .
3.Using mechanical pumping from an individual soak pit of your own.This too has maintenance of pump frequently (at lest once in two years )
We had installed a industrial strength PVC gate valve and it has been working fine (touch wood) till now. We have done this almost 7 years ago and we use the valve regularly during heavy rains.I went with a PVC gate Value to manually stop the water from coming in. So far I have not had to use it. PVC gate vale is non-corrosive.
When the showers start becoming heavy, we close the valve and wait for about 12 hours before re-opening in
It was an expensive fix, but we definitely needed it and has been worth it.
Raising floor of Bathrooms, increasing threshold drainage height are temporary measures because a heavier rain will still leave a mess.
Hope this helps.