We are going in Feb, so looks like we have to do it before. While there is a lot of evidence that you need to the Singapore one multiple times. I couldn't find anything that said you need to do the MDAC for every arrival into Malaysia and whether you could just put the end date of the final Malaysia port. I'd be really appreciative if you could let us know what happened after the cruise.
I also wondered if cruisers were exempt from the Malaysia form as there is virtually nothing on any of the cruise critic threads. I've just posted a general question on the 'other cruise lines' thread as it reaches a wider audience
RE: no internet, I'm guessing TUI will provide basic WiFi capability for this, but it is likely to be slow. If you have a fairly modern phone you cam buy something called an e-sim (a bit like a data sim card but it is electronic rather than physical). Altough you cant use it at sea, you can get 5Gb for all Asia countries for about £10. Martin Lewis moneysaving expert explains them here
https://www.moneysavingexpert.com/mobiles/what-is-an-esim/