Yes, Tom, we all kill them with kindness at first. Or we forget them for a couple of days and they shrivel up and die. One way or the other.
The road to bonsai glory goes past a pile of dead trees... somehow, we all need to kill a few.
Anyway, as far as the bugs, you would probably see them. You might notice changes in the leaf shape, or tunnels inside of the leaves themselves. You might see scale, which looks like little hard oyster shells on the stems or branches. You might see little aphids, or wooly things.... but most likely, with careful monitoring, you would see them.
10-30-30 is pretty high for submerging, so that might have been a problem? Maybe? Some other member may know better. The basic theory is that if the fertilizer salts are saltier than the water content inside of the roots, the roots will send water out instead of pulling it in. That's a dumb way of describing it, but I'm really tired tonight and don't seem to have the energy to do better.
But the bottom line is that if there is too much fertilizer in the soil, it dries out the roots. Simply watering thoroughly, washing away the fertilizer, is the safest way to get rid of the ferts.
Even in the best case scenario, elms will struggle to live indoors. You can do it, especially with grow lights and humidity trays, but they will never thrive like they do outside. If you read threads about growing indoors, you will find more information. A cheap fluorescent light, hung close to your trees, may be necessary for their long term survival.
Keep asking questions, we all do at first.
And don't be too worried about killing your first trees... bonsai is an addiction, and eventually you learn how to keep trees alive, then how to carve them, then how to show and enjoy them to the fullest. It's a long road, but worth walking.
Joanie