re: Which is the correct resin to use?

I think that Total Boat is so widely used in demo videos is that they sponsored many of them.  If I am thinking about the right company, they developed a deep pour epoxy specifically for river tables and had a big marketing campaign to get the word out.  

I am certainly no expert but I think nearly any epoxy will work.  The main consideration is how deep the cavity being filled is.  If your BIL is going to be routing out the cavity he can control that.  If for example he keeps the cavities 1/2" or less, he will not have any issues with curing or overheating as it cures regardless of which one he uses.  There are some that are harder than others and for cutting boards he should look for ones with a high hardness rating but they are all going to show wear from knives.   I filled a cavity that I routed out on a friend's live edge table with this Shabebe resin.  It is pretty inexpensive and worked very well.  It did take about 48 hours to cure enough to smooth and polish it.    I limited most of the cavity to about 1/2" but it had a void that was over an inch deep in places.  After it cured I used a LA block plane to flatten it down to near the surface level, followed by a card scraper for final leveling before sanding and polishing.  

PS.  I suppose that there is one other consideration.  There are actually food grade epoxies for use in commercial kitchens but that is probably not necessary here.