1. The screen brightness changes to very bright sometimes, when coming out of the unlock state.
Are you using an app like Juice Defender? If so, adjust the sensitivity of the Auto-brightness to slower. If not, change brightness setting from Auto to a fixed brightness that suits your needs.

2. When walking, with the phone in my pocket, if I receive a text the phone will on its own unlock and begin opening other apps.
May be harsh as an answer... get a case that clips to your belt. I never have issues since it is always in my case.

3. USB mode is very inconsistent. Sometimes it will work, sometimes it won't. After about 6 connects and disconnects I can then move files around from the phone to my MacBook.
I do not have issues with USB connectivity. When I select USB mode it simply connects.

4. Sometimes, when texting, characters that I am typing do not appear. To get the messaging app to work correctly I need to shut down the app and reopen it.
Are you using the default app? Are you using the default keyboard? If so, I can't help. The Telus version works flawlessly with the default KB and text app.

 

If you aren't using a lock screen, you can open the messaging app by swiping the envelope that shows up on the screen when you have a new message. It may be opening that in your pocket and then going to who knows what.

I have the same issue with #5. I don't have decent access for any event I go to where there are a few thousand people or more.

 

I have noticed some of this too and I guess here is my input. The brightness of the screen seems to be an interesting weird thing. The screen is either too dark or too light for the environment until I press the power button the screen turns off and than turn the screen back on and the light sensor adjusts the brightness properly.

Second I have never had the situation where my phone would power on and unlock in my pocket and call or activate different things. I have a simple hard shell case which is to protect my phone when it bounces off the ground. I am using the default unlock method where you have to swipe the screen aside to unlock the device.

USB mode - I have found that you must update windows media player and keep it up to date. The environment that Windows Uses to mount the USB MTP device is contained in the Microsoft Windows Media Player package. I have a special distribution of windows 7 (Windows 7 N) to be exact and until I installed Windows Media Player (latest version) I was not able to connect both my phone nor my tablet. So keep it updated and restart your phone every now and than - since that time I have not had any problems with USB mode.

For texting I have found that the stock keyboard is not very good! - I actually hate the stock samsung keyboard and I think I want to burn it. Horrible word complete horrible key structure and just plain bad. Best one I have found is Swift Key X - just magical great complete great for people like me with very big basketball hands big thumbs that press multiple buttons at the same time. Horrible for spelling out names because autocorrect really want to make it something normal. I have misspelled my bosses name in 3 consecutive messages so that was fail! Swipe works well but all that swiping action is a little too much for my brain when I am under stress I would much rather frantically press all the buttons in the word instead of going crazy with the swiping action.

As for the coverage issue I have never experienced one - went to Europe with a connecting flight in Germany and T-Mobile worked just fine on the Airport at Frankfurt as well as Munich. Went to Istanbul had no problem with connecting to the roaming on the local carriers - went all over eastern Europe with my phone no problem connecting to the roaming towers nor putting in a local carrier sim into my phone which got unlocked by T-Mobile.

 

OK, I haven't had any of these problems so I can't really address all of them.

However, you might want to try a different MicroUSB to USB cable. I have heard of a few people having trouble with connections related to cables. I regularly connect to my Mac Pro and MBP as well as many iMac where I work, I've never had a problem with any of them, using the aftermarket cable I carry with me.