The first criteria as you mentioned is the atributes of the item. You can't imbue with Speed any item that has Speed in their atributes, no matter if its increase or decrease your Speed.
I've been testing the other criteria and here is my tests:
All boots with NO required level and of course no Speed as an atribute can be imbued with Powerful Swiftness. For example:
- Leather Boots

- Coconout Shoes

- Pirate boots

- Golden Boots

- Steel Boots

And then we have all these boots with Required Level that gives NO Speed but still cannot be imbued with Powerful Swiftness. For example:
- Magma Boots (Lvl 35)

- Glacier Shoes (Lvl 35)

- Lightning Boots (Lvl 35)

- Terra Boots (Lvl 35)

- Guardian Boots (Lvl 70)

Then we jump to those boots with required level that gives NO Speed but CAN BE IMBUED with Powerful Swiftness. For example:
- Pair of Nightmare Boots (Lvl 140)

- Pair of Dreamwalkers Boots (Lvl 180)

- Pair of Soulwakers (Lvl 400)

In conclusion:
The criteria to imbue with Powerful Swiftness are:
- Boots with no required level.
- Boots that gives no speed with required level 140 or higher. (Currently -27.Mar-2023- there are no boots with required level between 70 and 140 with an empty slot, that's why I say 140 or higher.