I would do both of your suggestions and leave it staked for 1 year or so, gradually loosening the stake from the tree after 2 months (putting in soil is obviously done before the staking). - short version

Basically, I would loosen the soil around the tree, get it square, add soil where needed, backfill, hammer 3 stakes in the ground equally apart from each other and at least a foot from the planting hole in undisturbed soil, attach staking rope/wire/string whatever as long as it doesn't cut the tree (think cut garden hose softness), leave for around 2 months, when relatively stable gradually loosen staking mechanism over the course of 1-2 years depending on your soil and wind exposure, then enjoy tree until it dies. - long version

ok newbie.. listen close...

if you would have bought a 3 to 5 footer [and paid $100 less] .. in about 3 years.. it would have been the size you have now.. you would NOT have had BB problems.. and it would have been less affected by the wind ... if at all ..

buying big.. is often a fools errand ... trust us on that..

if it fails .. go smaller .. its easier all around ...

this tree should have been planted at least 15 to 20 feet from the house.. how are you on that???? .. if there is a next time.. check here first ...

as to remedy ... 'established' trees need to blow in the wind.. but recent transplants need to be stabilized so root can grow and attach it to the ground... otherwise roots will keep breaking ...

how big a stake setup you need.. on a 15 foot tree is unknown.. especially w/o a picture ...

good luck

ken

PS: i got interrupted by a play date arrival .. and have totally lost my train of thought.. if that train ever comes back to the station.. i will chime back in ... dont hold your breath.. lol ... chuga, chuga.. choo choo ...