Since "buttons/submit.tpl" uses "buttons/button.tpl" have you tried using that instead and adding "button_title=$lng.lbl_submit_1 style="button" directly? You would have to make language variables for lbl_submit_1, lbl_submit_2, lbl_submit_3, etc. but it should get what you want although it may actually be easier to make buttons/submit_1.tpl, buttons/submit_2.tpl etc. which you may have to do anyway. I hope this helps... although I have my doubts
I just made $lng.lbl_submit say "Submit and Continue"