Your numbers are all a bit odd (perhaps you are using a playtest edition, or your DM is providing his own customized equipment list), but I think your question is answerable with the information you've provided.
Why would the scythe be considered heavy?
Heavy vs. Not Heavy
The Heavy keyword isn't necessarily tied to weight.
Heavy. Small creatures have disadvantage on attack rolls with heavy weapons. A heavy weapon's size and bulk makes it too large for a Small creature to use effectively.
Compare the great club, which weighs 10 lbs. and is not Heavy to the glaive, which weighs 6 lbs. and is Heavy.
The difference isn't their weight. The difference is that a small creature is going to have more difficulty using a glaive several times longer than they are than they will a great club.
Scythe vs. Longsword
Why is the scythe so much "worse" than the longsword? Because the longsword is a Martial Weapon and the scythe is merely a Simple Weapon.
Simple Weapons all tend to be a bit worse than their Martial equivalents, but more characters are proficient with them. Compare the mace to the longsword... You lose damage and the versatile keyword. But Bards, Clerics, Druids, Monks, Rogues, and Warlocks are proficient in the mace but not the longsword.
If you are one of these classes, you are likely to prefer the mace, simply because you have an easier time hitting your target.
Could a warlock still use a scythe?
Short answer: Yes.
Any character can use any weapon, as spelled out in this featured question.
Warlocks are proficient with simple weapons, which means that they can add their proficiency modifier to attack rolls with simple weapons. This would include the scythe.
The fact that the scythe is two-handed is irrelevant in this case.
If you are a Pact of the Blade warlock, you can use any weapon as your pact weapon and be proficient in it. This means that you can use the scythe, but you would be better off mechanically choosing a Martial Weapon of some kind.
The only penalty you need to worry about is the penalty from Heavy. If you are a small-sized warlock (e.g. halfling or gnome), you would have disadvantage on attack rolls with the scythe. This is regardless of whether or not you are proficient with the weapon.
Appendix: Spellcasting with a Scythe
How does spell casting with a two-handed weapon work?
The rules for a two-handed weapon state that it only occupies a second hand while making an attack with it.
Two-Handed (p. 147). This property
is relevant only when you attack with the
weapon, not when you simply hold it.
(from this answer)
You can freely cast spells with Somatic components, as you have a hand free.
Spells with Material components depend on what you are using to satisfy the Material component.
Arcane Focus: If using an arcane focus, you will need to move the focus in and out of your free hand. This will require an item interaction, which you are allowed one of per turn.
For the most part, this isn't much of a bother. You use your item interaction to equip your arcane focus, and cast your spell. The next turn you use your item interaction to stow the focus, and then attack.
Your main loss is that you can't make attacks of opportunity using your scythe on the turn after you cast your spell. However, if your arcane focus is a one-handed melee weapon (like a staff) you can use that to take opportunity attacks.
You may also run into issues with casting bonus action spells and attacking on the same turn.
One other possibility would be to ask your DM if you can get a scythe arcane focus. This isn't RAW, but is unlikely to be game-breaking.
Component Pouch: The rules say that a spell caster "must have a hand free to access a spell’s material components."
If you have a lax DM, then they can interpret this as "you have a hand free, so you can access the components in your spell component pouch." You can cast and attack without penalty.
If you have a strict DM, they may interpret "have a hand free to access a spell's material components" to mean that you must have the component in your hand.
Either way, you can use the item interaction pattern listed in the arcane focus section to perform your spell casting.
Note that manually tracking and using components will generally function the same as a Component Pouch.