I feel they should travel/enter on the 4-5 weeks tickets.... as mentioned in the visa applications
Do prepare your in-laws to handle various types of questions that they may/may not face at immigration..... so that they do not panic, get nervous or flustered.... this can happen quite easily to elderly parents who are travelling alone.... they should be very confident + careful about what they say
Also, they should carry supporting documents and evidence.... including your host/their documents + some amount of proof of funds (do not keep money/travelers cheques in checked-in baggage), original travel medical insurance policies, medicines required for the travel along with doctors' prescriptions (rest of the medicines can be packed into checked-in baggage), etc
When they answer confidently, they will most likely get the regular stamp (without any date to leave under the stamp).... this will allow them to stay for up to 6 months.... you can then re-book their tickets later depending on whether you would like to extend their visitor status beyond the 6 months