As far as the barn doors go, I do not believe an interior handle was ever an option, as passengers were never intended to sit/ride in the cargo area.
As for the AC lines being "blocked off": This could have been done for a number of reasons, and done a number of ways. The most logical reason is that the rear lines developed a leak and it was cost prohibitive to replace the lines. Several companies make universal block off plates for vehicles with rear AC for this very reason. It cheaper and easier to just cut the lines and install a $30 block off plate kit to get the front AC going. You'd have to inspect and follow the lines to see where and how they were cut and blocked off. And then there's the unknown, like mentioned above. What if the rear expansion valve is also bad? What if the rear evaporator is full of trash/debris ?? You could do a ton of work replacing the lines only to find over $1000 in more parts are needed. Personally, I would just leave it with just the front AC unless you know the previous owner and he can tell you EXACTLY what is wrong with it and what all was done to it. Otherwise it's a huge gamble in my opinion. Money would be better spent elsewhere. You already mentioned that you will pull the rear seats as they will never be used, so if nobody will ever sit back there, then it's not worth the time, effort, hassle, and MONEY to try and get the rear AC going. Just my 2 cents.
As for the cruise control, could be a number if things. First off I would check all fuses. Second, check ALL brake lights, bulbs, and the brake light switch at the pedal. If ANY of those are acting up on a GM vehicle, the cruse control will not work. Also, if you have LED tails, or LED bulbs in your tails/brake lights, the cruise will not work. I that one from personal experience.
As for the grill, I wouldn't bother trying to JB weld it especially if you already have a replacement available for only 50 bucks. The JB weld will work temporarily, but the grill will eventually crack again, as the crack will usually end up spreading beyond the repaired area. I honestly would just spend the 50 bucks and be done with it, as long as the replacement grill is in GOOD condition.