I want to book a cheap vacation package through tripmasters. One of my options is a UA flight, which I can either get as UA direct or as an LH codeshare. I know that UA's basic economy doesn't even allow for carry-on bags and there is no seat selection. Tripmaster notes say this:

Seat selection with code-share partner:
When US Airways sells a United Airlines flight, they do not allow seat selection.
When Lufthansa sells a United Airlines flight, they do allow seat selection.
When Air Canada sells a United Airlines flight, they do not allow seat selection.
When Copa Airlines sells a United Airlines flight, it does allow seat selection.

I am concerned this is outdated (ahem, US Airways?) but don't quite understand the 'selling' part. If it isn't outdated, would this mean that I would get to choose my seat if I booked the LH codeshare? I assume I won't booking UA direct.

I hope this is the right place.

Also, I have a question about the return, which could be on Brussels, LH or Swiss. Which has the best cheap E?