I have roughly 1,000 sq. ft. of 3/4" x 2-1/4" red oak. About 750 sq. ft. (3 bedrooms, and an L-shaped living room/dining room) had existing hardwood under the carpet at the time of purchase. A family room and a connected hallway did not have hardwood. It did, however, have another layer of subfloor on it that was either 5/8" or 3/4" (it still sat a little lower than the bedrooms with the existing wood even with the extra subfloor). This was ripped up and new hardwood was installed in this area and it matches the existing rooms height.

The house was built in 1974 and has an open basement with 2x10 joists that are 16" on center. I believe everywhere there is wood throughout the house, both existing and new, there is only a 1/2" plywood subfloor underneath. There are some spots where I can see when being built the nail has missed the joist when fastening down the subfloor. When the new wood was installed, the installers used 2" staples. The existing wood was installed with cleats, and for whatever reason, someone hammered the cleats back against the subfloor from below so they no longer point on a downward angle, but instead are bent back nearly flush against the subfloor. I am not sure if they nailed into the joists or not with the cleats. The perimeter pieces were stapled in place on the existing wood. All hardwood is installed perpendicular to the joists.

I cannot tell if the existing hardwood was original or not, but it has certainly been there for a while. The floor is VERY squeaky and creeky and there are some pops as well. I do recall it being very squeaky under the carpet as well. I can go in the basement and press up on the squeaky areas and reproduce the squeak from below. The floor "feels strong" for the most part, but there certainly is a little bounce to it compared to other floors I've been on. And sometimes when stepping in one area, you can hear squeaks in another area. The seams seem particularly "weak", I am not sure if it is tongue and groove.

My questions are:
1. Do I have anything to worry about long term? Will the floor eventually fail? My thinking is if the existing hardwood is still in good shape and has probably been there the majority of the house's life (built in '74) it should continue to last? From what I've read 1/2" is not preferred at all, and really 5/8" T&G is the min.
2. What can I do to lessen the squeaks? Is tapping wood shims in between the joist and the floor where the squeaks are a good idea? If not, what can I try?
3. What can I do to reinforce the subfloor AND lessen squeaks? Would I benefit from blocking in-between joists at all? Is blocking something a DIYer can do, or is there a science to it in terms of positioning where the blocking should be?

Any input is appreciated. Thanks in advance.