Hi,
Sometimes the missions aren't activated and don't count your progress until u click the complete button next to them. So you should try doing this and then playing and checking whether your stats are updating or not. This is usually the reason so I believe this should solve your problem.

Yeah 50% off on upgrades n speedups prolly.
u can check the festive season from christmas as it has all the sales info.

U can try asking the support team to unlink it if possible by asking them through help@tankionline.com.

If you are talking about upgrading from a mk to a higher one or microupgrading it then you have to pay since you are upgrading or making your turret stronger which is kinda obvious but if you are asking about something else please give proper details in you question

U can report such issues to help@tankionline.com with proof of course as no "gifts" are given to the players for such .