If she accepts the position of duke, will she still be able to marry Albert, who also has ...

i hope for her sake she doesn’t since by being the future duchess and marrying albert she will have to take care of both estates him as well it would take a toll on her

Maybe they can keep both of their titles plus Albert is a relative of the Queen so he has a royal bloodline. So their title in the future might be so majestic.

I'm not sure. One of the earlier plotlines was that Avery's father had to break his previous engagement because his fiancee refused to give up her title. Apparently if both parties are heirs to an estate, one of them has to give it up for them to get married, or else one family could just collect all the land.

If they merge the estates I think there's no problem, and technically, Avery would be the one with the higher title, so Albert will become Duke by marrying her... But I feel like Avery will leave the title to his cousin (not the one who kidnapped her, the other one, the one she met with the princess)

I was thinking the same thing about leaving it to the cousin

In reality, would be no problema in getting more than one title. Actually the majority of royalty have more than one title. The only consequence would be that their first born would be an archduke, which is pretty sweet