Would cutting it to 2.5 meters be beneficial so it would be at an even harmonic of the ham bands
No, for most of the bands you've listed. Dipoles are resonant around 1/2λ and odd harmonics thereof: 3/2λ, 5/2λ, 7/2λ, etc. In other words, the dipole needs to be a half-wavelength or longer to be resonant.
But do not fret. A dipole that's too short just means a poor match to the receiver, which may or may not be an issue. See What is the relationship between SWR and receive performance?
Also If I make it a dipole style antenna, does the other element have to run in the opposite direction of it, or can it run parallel to it
The two halves can not be parallel. If you just had two parallel halves of a dipole, you'd have a twin-lead transmission line. Transmission lines are designed to not radiate or receive, so they make horrible antennas.
But the other half doesn't need to be exactly opposite, either. They just need to go generally in different directions. See the inverted vee antenna for a very common way of building a dipole. The arms of the dipole don't need to be straight, either.
Generally, my advice for someone in your situation is this: Wire is cheap. Don't fret. Try science. Any HF antenna you try to fit inside is going to be a compromise solution because there simply isn't enough room. Any indoor antenna is also going to pick up a lot of noise from electronics in the house. Theoretical antenna models don't account for that. So just build an antenna, get on the air, and experiment until you find something that works well in your specific environment.