No you can not drill holes in the bottom of the pit. I have installed many 30" by 30" and 30" by 36" pits. The extra width helps with how long it takes the pit to fill allowing the pump the proper rest time between cycles. Why get cheap all of a sudden? As you said you spent a fortune on the overhead sewer conversion, why cheap out on the sump pump pit, and have your basement flood out again for not having a properly sized and installed sump pump pit?
There is a formula to figuring out the size of a sump pump pit. I don't have it in front of me at this time, but it has to do with total square footage of the roof, property, and the 100 year rain fall numbers. 99% of the homes built use the 18x24 pits and get ok use out of it till a good rain hits the area. Then the pump is over worked, and burns out. Some people even with a small rain the pump gets overworked and burns out due to short cycling.
What I would do if I was in your place and wanted to meet what the inspector wants, is ask the inspector what he recommends.