You can’t upgrade mini-sentries, and as far as I know that sort of glitch doesn’t exist. There is, however, a glitch where a mini-sentry appears as a small red level 3 sentry. Must be some sort of model mix-up.
Mini-sentries set up really fast and can throw your enemies off, giving them two targets shooting at them to deal with. Plus it’s quite good for assists and a quick area denial. Mostly for offensive engies, which is why you get +25 health with the Gunslinger.
Anything can be moved now. Level 1-3, any type of building, can be picked up and moved. Sets up faster than first time. The drawback is you run slower while carrying a pre-built building.
People are saying Wrangler is OP, so that may need to be balanced. I’m not certain myself because I haven’t gotten it yet, but it does seem like it doesn’t have enough drawbacks to balance out it’s strengths.
Frontier Justice is cool too. I was worried it would be OP but it seems to be pretty balanced. Works well with the Gunslinger, because your sentries go up quick, get a few assists/kills, go down, and then you have crits to shoot. And if you want the crits right away you can just blow up your sentry and you won’t feel bad because it was cheap and fast to build anyway.