In some updates; Virtuemart Developer Team change the core of Virtuemart that effects the front-end side or add some further tweaks (mostly JavaScript tweaks) to make Virtuemart a better eCommerce software. This is what updates are for. Once they change a thing that effects the Virtuemart functionalities in the front-end; then the templates that are developed for it; needs an update as well to support that tweak. This is absolutely normal.
You can't say CleanMart is "unstable" for this situation. That would be totally non-sense. If Virtuemart Team decide not to touch things in the functionalities in the front-end, then CleanMart will live forever and ever., without requiring any single ongoing update.
You don't have to get the updates the moment they are out (unless the update patches a critical security bug) In this website, flexiblewebdesign.com, we are using the modified version of Virtuemart 1.0 (not even VM2 nor VM3) and everything has been going just fine since 5 years. Updates are not the "best" some time.
About the problems you have, you should also check whether your website has a jQuery conflict problem or not.
- See the browser console and check if there is any RED Error Message
(not working modal popup seems like a jQuery conflict)
jQuery Conflicts or TypeErros prevents all JavaScript based functionalities to not work.
Can you try "jQuery Easy" joomla plugin? It is a free plugin (
extensions.joomla.org/extension/jquery-easy) It is usually the best for resolving jQuery Conflict with couple of clicks (after installing; select
jquery v1.11.3 and enable
jQuery Migrate from the plugin parameters)
If still no luck, provide me your website address and let me see it.
- About the payment method not appearing problem. You should also check if you have any "country" restriction for the payment method you published. If you have a selected country for the payment method, then this payment method won't be displayed until the user logged into website and has the country address that matches with the payment method's country.
If stil no luck, then disable the CleanMart temporarily from your website and see how ordinary Virtuemart behaves.
To temporarily disable CleanMArt from your store, follow these 2 steps;
1 - Rename this folder:
/templates/YOUR_TEMPLATE/html/com_virtuemart/
to
/templates/YOUR_TEMPLATE/html/com_virtuemart2/
2 - Unpublish this joomla plugin:
"Flexible VM3 Template"
Regards.