Once you start to research possible infringements and someone is able to find out about it (e.g. because you asked in a public forum) every violation might be considered willful infringement. Patent infringement - Wikipedia. The point is: willful infringement might lead to higher fines.
I don't want to do anything that might fall even within a grey area
In this case you can forget it right from the start. The issue is not any wii patents but any motion tracking patents. Once you start to search for motion tracking patents you will find soon that you can not develop anything at all if you want to avoid anything even remotely "gray".
IMHO nobody would sue you unless you start making money. Patent trolling is a business so they will not troll until there is sufficient expected profit. Most probably you will be to small to be of any relevance. However if you should really make money then you should pay an attorney to take care of this.