no expiry if you have usage in 90 days.

Yes, you need to make call/sms/use data.

Yes, just normal sign up will do. 

One way to find out is to see whether Unifi@mobile is on Bands 3 or 7 of LTE.  If it is, then it's using the Celcom 4G network.

confirm yes..
mine deduct 2MB for playing 1 games in chess.com from 3G/LTE quota and the phone shown 4G connection..

I guess you can purchase it now so it should be extend the timing, right?
Look like it is.. Perhaps you may try manually switch to celcom LTE and streaming youtube, see whether the 3G/LTE will kena deduct data quota or not.

Not need to mention about the unlimited pass RM55.. That is exclusively festival promo..

But if this promo without a free weekly again, the monthly cost is RM80..
At least save rm20 on that particular weekly better than nothing..

Of course, need to fully utlize 101% non-stop internet then it's more valuable for it.
You use what device for your unifi mobile?
Maybe after you finish 3g/lte quota (that column, but NOT another column with only LTE), your data totally cannot use anymore.
Unless you change network to Unifi mobile, then it will consume the data quota of LTE ONLY.