It should be YOUR IMEI, of the device you own, not anybody else's IMEI
I compared the LTE bands (no 5G for the 6mq) and their "Light speed" offer should work, since the 6mq supports both of the required LTE bands (4 and 12).
Their "Warp speed" needs band 4 and 13, the latter is not supported by the 6mq, so, there might be speed and connectivity issues.
The shop site also mentions "short trips" (e.g. vacation) should be fine, but if you want to use the 6mq for a prolonged period (e.g. living there), there might be issues with the FCC, missing certifications, etc. and your provider may decline service in the long run.
At least that's how I understood it
For example:
https://compliancetesting.com/fcc-certification-for-cell-phones-guide/
Also: You'd still need a physical SIM card, the 6mq doesn't have a dedicated eSIM chip build in.
Like Lhotze said in your LTB post, somebody is offering a 6mq right now, but maybe make sure which display technology the phone currently has, after the seller exchanged it last year (depends on "when" it happened(?). Early last year they might have had OLED displays still, a bit later the same year they started selling IPS only, which seem to have huge battery drainage..!)