Looking for recommendations: mounting tape, semi-permanant (12mo)

The 12 months part is what makes this very challenging.
Check out Velcro - removable mounting tape. Check the different strengths, i.e., "Holds 2lbs","Holds 5lbs" etc.
Check with Cody Martin on his offer for Orafol tape. Especially the removable part.

 

3M Command Strips, available pretty much anywhere.

 

As Moze recommended, the 3M Command Strips are definitely worth checking out.

 

I also recommend 3M Command Strips. Get the large 16-lb strips and put one in each corner. (Maybe overkill for 12" x 18", but that's what I would use.) Easy to work with. I burnish them onto the panel with a wallpaper seam roller. Bonus: they have two parts, stuck together like velcro, so if you don't get the panel up straight on the first try, you can rip it off and try again.

They'll stay up for months, and come off easy and clean when the time comes. Be sure to follow the removal instructions to avoid pulling off paint. Pull them down, not out. A pair of pliers is helpful if they are stubborn.

We use them all the time to hold up Gatorfoam and Sintra panels for temporary exhibits (6 months to 18 months).

Sam's Club has the best price on them in my area.