Hi Walrus, welcome to the forums. "Only" and "just" have many meanings... Among all those meanings, "just" can mean "only", yes, so there wouldn't be any difference in certain contexts.
Thanks. Anyway I know it is not true that "only" modifies what's close to it, but it modifies what's stressed with the voice. As for the split infinitives, that's a prescriptive rule, and a lot of native speakers split them all the time. I have to say that split infinitives sound good to me.
The problem is that I don't know what most natives usually do...
I tried to install the software in the main PC. - If I want "only" to modify "the main PC", I can say --->
I only tried to install the software in the main PC.
But in sentences like the one I posted earlier... it's difficult because there is "even" that is already before the main verb. Also, the verb "decide" sounds a little bit differently to me, and I don't know if moving "only" before it would be a good idea.
Thanks again.