1. I am about equal on solving vs. creating problems by using the forward assist. Worst case scenario, you can use your thumb against the BCG cutout. Not necessary in my opinion.
2. A chrome lined barrel adds lubricity and can enhance reliability in tough conditions (typically a seriously hot chamber). However, your chances of running the rifle hard enough to notice that problem outside of some carbine training classes is extremely low. I've read tests of an unlined barrel shot continuously throughout a single weekend - the throat gave out just a bit before 10k rounds. Considering that is about $4000 of .223 ammo, I'd guess an unlined barrel is not an issue for you if you are looking to save a few hundred on the rifle.
3. The AR15 is one of the few rifles where your gas chamber is also your bolt carrier group. There are very few rifles that deliberately expose the gas chamber to dust and debris and there is probably a good reason for that; however that is part and parcel of the AR15. I do consider the dust cover to be necessary, though ARs certainly can and do run well without them, especially with generous lube. If I had a Sport, I would consider investing in a product like the Brownells Dust Cover/Safety Flag to minimize the amount of debris that can enter through the ejection port