Changing the labels in that is part of the base plugin, do you mean what appears on the back end or the front end? I am not sure where your issue is coming from. I just deactivated all my add-ons (except propack and tagalong like you have), I went in and changed theExperience/Results/ label for phone to MYPHONE. saved changes, went to front end, voila it is MY PHONE in results. So I am not sure what is causing you to have problems on your site I am afraid.
The developer of Store Locator Plus understands all too well about abandoned WP lpugins and that is how he got into the WordPress world to begin with. (about his background) . That is why he and many others have offered Paid Add-ons that need to be purchased. Otherwise it is just a hobby and people tire of hobbies. The intent is to be able to continue providing updates and maintenance and support, to attend WordCamp conferences and keep up to date on WordPress changes and new developments.
As far as the WPML issue, that again was a change in WP codex to change to text. That was a while ago.
On another note, anyone who does want to translate will get a free add-on or discount to new add-ons. More info on that. can be found on our new docs site.