Lopi Leyden combustor question

After seeing what I thought was a piece of metal in my combustion box in the rear of the stove, I have a new combustor here and have removed the old one. The piece that looked like metal is actually a piece of the combustor brick that broke off the inside.

My question now is, with the combustor out, there is a square hole in the rear most part of the stove that I can look through and see the floor. Along the seem directly behind the combustor brick some of the stove cement came out when I ran my stove vac through. Is it important for that seem to be cemented with a 1 1/2 x 1 inch hole there anyway?

Another question... of the three bolts that hold the rear bricks in place, two of them broke. By mustering all the patience I have, i was able to drill them out, run a tap in the holes to clean them up and get some new bolts. They are just 1/4-20. I only have grade 8 here. Would stainless be a better way to go? Im thinking if I had to replace the combustor after one year, chances are I will have to do it again. Id like to put in bolts that I will have a better chance of getting out without breaking next time. My thoughts are, grade 8 and stainless will be a real PIA to drill out should I have to. But grade 5 might be more prone to breaking. What happens to stainless bolts after they have gone through countless heat cycles?

Have you talked to the dealer you got it from? He might be of some assistance, since it's only a year old.