1. We don't support J2.5 any longer and neither do Joomla. You would be wise to upgrade to J3 as soon as you can because J2.5 has a lot of security exposures.
2. It it worth trying to update to latest Gantry 4 yes - but backup first just in case.
3. There are several items that are causing mixed content issues so you should speak to the developers of those extensions (who will tell you to upgrade no doubt). These include mod_susnet, jf_social, event_booking.
4. It is most likely one of those third-party extensions loading that jquery - see 3. above.
Regards, Mark.
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