First time installing a solid wood floor, and in my prep work, after pulling all the carpet, i've noticed there are quite a lot of variances across the first room.

Is this normal and part and parcel of installing in a home that was only built in 2009 ?

I'm really not entirely sure what to do to fix. Perhaps spend a lot of time with a nail set, then renting a sander of some kind ? My level in places is rocking between 1/2" to 3/4" of an inch.

Don't sand or plane more than one-third the thickness of your sub-floor.
Then build up the adjacent area with plywood, roll roofing or latex underlayment mix, depending on what type of flooring you are planning to install.
This is common after the joists have dried out. It could have been fixed before the sub-floor was installed, in most cases. It would be a big problem to take the sub-floor out and plane the joists if sub-floor adhesive was used.
You want to get within about a quarter-inch in six feet.

That pic is misleading. What's it look like with a string line pulled tight over the area from one side of the room to another?

Is there a hump to the right of the level? Is this the typical problem that you have? Then build up the 'dips' with multiple layers of roofing felt, shingles, etc. ala a topographical relief map and has more filler material in the middle and gradually feathers out toward the 'humps'.

The hump is more at the center where the it nails into the joists.

I was wondering if i should just pull up the plywood, and planer down the joists so that they're level, and then re-lay some new plywood... or if i can somehow get those boards up without damaging (somehow doubt that ! ) then re-use those.

One hump? Do the feathering buildup. It will be a lot less work and your finished floor will be fine. Also, there should also be construction adhesive in addition to the nails securing the subfloor to the joists. Plus if the underlayment is tongue and grooved you're going to have to cut down the new sheets to fit them in....all while only having the floor joists to stand on.

I purchased a 4" belt sander, and sanded down the humps and added extra sheets of aquabar B, in the low dips..

It worked out more or less fine. I may get a creak or two in that particular spot, but you can't see any humps in the wood. Was a lot of work mind !

This is the first time of someone using Aquabar for building up a low area.
How many sheets do you use to raise an eight of an inch?

Second time. I built up a couple of low spots with extra pieces of Aquabar B.

I guess that newspaper would work just as well, if enough layers are put down.

I had the exact same thing in an upstairs bedroom. I used thin high density cardboard (thank you Post Office) and cedar shingles to build up the low spots.