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.
Concerning the Themes, keeping up with developmen etc. , that is totally a WP issue. It is coming from WP . Wp updates quite often and has had some major updates. We have no control over that and we cannot make SLp to work with every old version of various plugins and themes, so the developer does the right thing and updates to make it work with the current version of WordPress. WP updated to 4.5 and are using the newer version of jquery, they have advised everyone to update and have noted that some themes that were poorly constructed or perhaps didn’t use best Practices were forcing javascript versions or Blocking jquery. They have a list of the bad boys themes and have advised users , if ther theme author that is from a commercial side wont update, perhaps change your them to a WordPress theme (their words) they also have no control of the free market commercial themes.
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.
Store Locator Plus is JavaScript intensive. The author wrote a blog about it here. and wrote about Themes and their function or non function a few more times , all the way back in 2012 Themes can and will interfere often with SLP loading depending on where in the schema it loads.
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.