I would only expect a modest improvement with the sleeve balun, particularly on receive.

The velocity factor only applies to the coax, you can ignore it for insulated wire. Solid inner insulator coax is almost always 0.66.

It may help to eliminate the twists and turns in the elements.

You may want to make separate antennae for 2m and 70cm.

You are line-of-sight to very very many switch mode power supplies and computers so there is going to be a lot of noise.

Is it better at night?

If you have a transmitter and an SWR meter you can compare SWR through the band to make a guess as to whether the antenna is tuned high or low. (Low SWR is good).

The cool kids use miniVNA antenna analysers these days, perhaps you can borrow one from someone at your local radio club. They come in a wide range of versions and quality.

There's not much activity on 6m here, just every now and again it opens up to remarkable distances, especially in the height of summer.

Can you hear any local beacons? They are always a good start, then try for more the more distant.

Leave the receiver on the output of various repeaters while you are doing other things, you never know when someone will pop up.

The dipole will do best broadside.

DXers prefer horizontal, while mobile stations prefer vertical.

Not really, I'm a bit out of touch with that. A search throws up lots of lists.

Beacons are almost all CW, so you'll need a receiver with a BFO or SSB capability to hear them, though strong ones may open the squelch on an FM receiver.