Hi there! I think you could definitely cut the top 2.5 ft and try to root it. As far as inducing branching, the only ways I know are 1) when it blooms it will usually branch or 2) when you cut the top part off if you cut above old leaf nodes on the bottom portion they will usually push new branches out of there. This will only do that on the bottom portion with its head lopped off, not the top portion you cut off.

Congrats on getting your plant to bloom indoors!

Please let us know what happens!

Deejvue,
You should have no problem, the bigger the cutting the easier (more likely) to root. So as big as your cutting will be it should root as long as you follow the general plumeria rooting rules.
As for branching, some plumies are know for branching better than others. I know some varieties that once it blooms, or if you prune/or sand down the tip, you may only get a single tip or 2 if your lucky, while others you might bet 5 tips. So basically depending on your cultivar and growing conditions, you may end with 2 trees in 2 season.

Good Luck
Happy growing.