Do we have to walk around in the shoes for 15 Mins or can we sit in them

You can sit in them but they work off of pressure. I wear them when sitting but find them most effective when walking.

Just bought these, says they can't be used with a pacemaker. Anyone know why?

I would address this issue with your medical doctor. But I will give you my opinion. Go online and look for a foot reflexology chart. It will show you that some of those trigger points do go to the heart and it just may be to much pressure and if you start to fill good you might use it for more than it states at 15 mins. a day.

I wear U.S. mens size 13. Will large fit me?

Hello, we have XLarge now, which fit for size 13

What are the ways to clean the sandals, please?

Generally, we suggest wiping by cloth. They are washable also! But please remember to dry the sandals in reverse side, to make the water inside of massage knobs leak out.