Is it acceptable to attach elevated deck joists to rafter tails

If code calls for footings, you need them for code compliance, at minimum. Typically code-compliant footings increase the odds that the deck remains level over time.

You will also want them (or more than the minimum) for resisting uplift if you are going to attach a roof (think of it as a sail or kite, when the wind blows.) As such a "mushroom" or pad type footing (where there's a concrete column/post and at the bottom of it there is a larger diameter pad) will help to resist uplift - be sure to use adequate reinforcing steel to hold it all together and tie in your anchor bolts.

It depends on your local building codes, but most places in the US, they will insist on the joist hangers. Joist hangers are far stronger and safer.

They are not that much more trouble, in fact they may be easier to work with. You can put them in half way, set the jousts in, then finish attaching them.