I have an iPad mini 2, with ios 7.2, can i manually upgrade to 8.1.2 and still jailbreak it?
No, you cannot. Apple is no longer signing the 8.1.2 update, as can be seen in the chart at The iPhone Wiki.



Edit: The screenshot got partially cut-off, but the upshot is that all devices that support 8.1.2 were listed under the same item, and all stopped signing on 10 February 2015.