To answer your first question: no, you don't drop Estrada for Lohse. Estrada has breakout written all over him this year, strikes out a ton of guys and is basically much better than Lohse. I myself picked Lohse up the other day, and dropped David Phelps. That's the range of player drop you need to look for if you want Lohse.
Now, your team. I'm assuming it's a 10 team Head to Head league. To give you a better analysis, draft rounds would've been nice, but I'll give it a shot regardless.
First off: you can drop Alex Avila and pick up either another position player or a pitcher (Lohse, for instance). In a 10-team/one catcher league, you do not need a second catcher. Especially not if you have a horse like Molina (nice pick there). Your infield is solid up the middle, and you did great by getting Ian Desmond to backup Tulowitzki for if/when he gets injured. Well done. As a result, Escobar is droppable/trade bait. You don't need three shortstops on your roster, and you don't want to play Escobar in UTIL2 just for steals, in my opinion.
I am personally a huge believer in Middlebrooks, so I like that pick a lot too. Having LaRoche start at first base is not ideal to me (not in a 10 team league anyway), but you could do worse. You could try to swing a trade for a better 1B, or pick one up off the wire.
Your OF is sick. Trout, Harper and Jones are 3 of the top 5 young superstar outfielders in the game, and they will give you everything you need: power, speed, RBIs, runs, the whole thing. Monster job there.
Starting rotation: You lack a trustworthy ace, but Dickey is worth a shot in that spot. Gallardo and Kennedy are "meh" in my book, but hopefully they'll perform. I personally like Sanchez and love Estrada, whereas Harrison is one of those "I don't know" kinda guys. RPs are solid. Nothing to see there.
Concluding: great offense, just minor tweaks needed. Pitching: I'd go out to look for a good/upper tier pitcher. Maybe package Avila and Escobar in a trade for a Doug Fister kind of SP + throw-in or something. Just don't drop Estrada for Lohse.