No way really to get a mantis to take to a burrow of your choosing. it's all chance really. Just a let the bug make up its own mind.
never ever hide a heater in the sand. Heaters have to be in open water preferably in a place exposed to high flow.
food: I find the easiest, cheapest food source is the grocery store's frozen section. Pick out any three of a myriad of fleshy marine foods. I feed my mantis table shrimp, silverside, and pacific little neck clams. They aren't picky but you really should cycle through three different foods; just shrimp and live doesn't really cut it nutrition wise... not in the long term anyways. You will learn how much to feed by watching the mantis; note how much it eats before it throws the food out; you'll also have to judge based on how big it is. It is also good to supplement the food once in a while with a supplement like selcon or vita-chem once in a while.
A few live hard shelled feeds a month is all you really need for a smasher. I usually go out once a month and buy a few turbos and hermits and stop feeding the mantis. just let him go to town for a week. If you feel like feeding live more often than that, go ahead. That said, P. ciliata is a spearer! you don't really need to buy any live for that guy; probably couldn't handle snails or hermits anyways. feel free to get the odd fish (must be significantly smaller than the mantis) or shrimp though. the mantis won't hate you for it.
mmm... if you keep a low water level, keep in mind the reduced volume. you'll probably essentially be dealing with a 20g at that point; plan the bioload accordingly. P. ciliata's minimum tank size is 20g anyways; footprint is more important to a mantis in terms of territory, so you're probably even better off with your plan than with a standard 20g. have you considered making the mangrove island into a pinnacle instead? that way you could fill the tank to its normal level.
and finally, P. ciliata is a highly color morphic species. Give a mantis a different environment and it will likely change color when it molts to whatever it thinks is most suitable. I believe the golden morph is usually caused by the presence of many hiding places and light. A lack of predators might help. Different cues= different morphs. More research could be done in this field. That's half the fun really though; get the mantis and just see what happens
if I get carpal tunnel, im sending you the medical bill!