Flooring – Joists have different height

I'm first time homeowner and bought a house (built in 1999) in fall of 2015 (Philadelphia suburb). During inspection of the house I noticed squeaky floors and inspector said its due to nails rubbing against the joist making the noise and its minor issue.

The other day I was in the basement (unfinished) to check floor sections which squeaked the most. Just out of curiosity I put 4 feet level at the bottom of the joists (across joists) and to my surprise there were gap between joists and the level (for the most joists I checked) – it range from 1/8 to 1/2. I don't see any sag on length of the joists but level is off across the joists. Joists are resting on sill plate and its in good condition (no rotting or anything). Joists are 2X10 (not engineered joist) and they do not span more than 16 feet. To me it seems like joists are of different height/depth. When you look at the cross section of the joist – it appears they have bend slightly (like forming letter C if you see the end of joist – but very slight). I don't see any signs of house settling, basement is poured concrete and no cracks or anything like that. Basement is currently dry but I'm not sure if previous owner had any moisture issue in the basement in past or not (although quite elaborate battery backup sump pump for unfinished basement may be due to some water issues in past, will find out in spring). Floor sheathing is 3/4" (i think plywood and osb). Most of load bearing walls (like second floor wall) tend to line up against perimeter of the foundation or the metal beam that passes through middle of the basement.

We started notice that some section of the main floor is uneven and we are observing similar issues on second floor as well (first floor and second floor unevenness do not match). I'm guessing its due to joists of having different height. In general house is leveled – inspector checked with level at few spots. I checked with 4 feet level and found some high and low spots on the floor (may 1/8 to 1/4 hi/low spots) and those spot run across the joist.

1. Is there any way to fix this? Is this a very serious issue – like house is falling apart – this is my first house so I don't know what's normal and what's very serious issue. I know foundation settling is a big issue and I ask inspector to check on it during house purchase process and he thought house foundation and framing is pretty sound (what ever his opinion worth…).

2. Most of the house is carpeted – so I was thinking about using those squeak end kit (where you drive the screw in joist and take off the top part). But with uneven joist – would it make the floor even more uneven? Is there any other way to stop squeak?

3. Carpet in some area is really bad and I thought I would install laminated floor in near future. But with uneven floor I don't think it will work well. what would be best flooring option for uneven floors (other than carpet and vynal)?

4. I'm not in finical situation to hire a contractor or a structural inspector to access the situation in near term, if this is a serious issue what time frame I should consider to address it?

Thanks in advance.