Does anyone understand the seatbelt/locking clip thing?

So tonight I went to dinner with my girlfriends and I promise I will get back to that later, but for now, I am concerned about the proper installation of carseats. My baby is 15 months old and I thought I had it down. Apparently, I suck. My friend was borrowing her Mom's car tonight and I noticed that the carseat base (rear facing) wasn't as secure as I felt it should be. I know what a safety guru my friend is, so I ran to my car to get my locking clip (since Mason wasn't with me). We installed it in her car with the lap/shoulder seatbelt. After the evening was over, I swung by Babies R Us to replace my clip and learned a something I didn't know about the locking clip. The lady told me to place it 1/2'' - 1'' from the latchplate (the part of the seatbelt that has the metal part sticking out). I had always put it on the opposite side of the carseat, assuming that the latchplate was sturdy once locked into the 2nd part of the seatbelt.

So, being the overprotective fanatic that I am I got home and IMMEDIATELY started researching it. I'm still very confused. I know my certain scenarios will come out as useless blabbering right now, so I'll try to pin point it to my specific problem. (Obviously, I am going to take Mason's carseat to a Certified Technician to find the exact way to install it.) My truck and car have lapbelts only in the middle position of the back seat and Mason has a forward facing carseat. The locking clip says NOT to use a locking clip with a lapbelt only. Oh great, my child has been at risk since I sold my Audi! From what I am reading, it appears my only option is a Belt Shortening Clip or Heavy Duty Locking Clip. Or, my other option is to twist my latchplate once and then clip it into place. That doesn't seem very scientific!

As for the lap/shoulder belt, it says that if your car has a retractable/locking shoulder belt, you also don't need a locking clip. That upon impact, your shoulder belt will lock and become tight enough to not launch your baby. I'm concerned about the 8-10 inches the carseat can slide side to side, when you aren't in an 'impact' situation. That can't be safe either, can it?

If anyone feels they know exactly what to tell me with a Lap Belt only option, please let me know. Otherwise, it seems I may have to order a clip directly from my vehicle's manufacturer. Here is where I'm reading all this mumbo jumbo.
CPSafety.com
CarseatSite.com