Well, let's see here...
I do wear a bra every day and have gotten to the point that I don't feel dressed without one.
It took me a lot of time and a lot of failed attempts to find the right bras to wear to alleviate some of the issues you discuss.
I have several different types of bras that I can wear with different kinds of shirts without getting discovered.
1- Yes it can get hot in the summer and I now fully understand why the first thing a lot of GG's do when they get home is to lose the bra.
2- Not really an issue.
First, find bras that actually fit and they are much less visible.
Second, I wear women's tank tops as undershirts. The straps are set wider than men's for the specific reason of hiding bra straps.
Third, I have a few different types of bras that I wear depending on the shirt I am going to wear; i.e., sports bras with polo shirts.
3- It can be an issue if you let it. Every other weekend, I have my son. During the summer when there is a high probability of us hitting the pool, I just don't wear a bra on Friday and any marks are gone by pool time on Saturday.
4- My go to is a soft cup bra I found at JC Penney. It fits perfect and comes in a A cup in my size which is rare. In other words, it actually fits. The adjusters for the bra are also in front instead of the back which is a huge help. I also have a few demibras/bralettes and several sports bras. As I said, My bra depends on the shirt I will be wearing.
5- Absolutely. I notice it more when I don't have my bra on than when I do.