I've heard folks have done that and had terrible problems with rough edges on the arms. I guess you need to cut the opening bigger.

Bad idea. Just buy the cheap one from Orca. It basically is the same as their more expensive one and works great.

I attempted training in various wetsuits/shorts and other nonsense before I did Casco Bay long course in 2017.

Get a S/R wetsuit if you are doing a S/R race, trust me.

My wife and I did Casco Bay 2 years ago. At the time there were very few options of commercially available wetsuits specifically for Swim/Run. And the ones that were available were expensive. My wife I both had our original QR LongJohns from the early 90s and an inexpensive Xterra full wetsuit. I cut the Xterra's legs and arms to make them shorter (you can do this to any length that's comfortable. I took the zippers from the old QR wetsuits and added them to the front of the Xterras. That was probably the trickiest part, but in the end it worked out just fine. I also then made some cool upgrades that weren't available to the current wetsuits. Like inside pockets for nutrition, and a built-in loop (front and back) for tow lines. I did use a sewing machine for the front zipper, and some wetsuit glue (available on amazon).

They both ended up working out perfectly! It was awesome having the ability to get the wetsuits off of your shoulders for some of the longer runs. I'm not sure it would have been possible with just a front zipper and the mandatory singlet. They may not be the prettiest, but they just have to work.

I say go for it.