It was only a date to aim for, that we would have everything in order by then (if that's possible.)
I would be ok if the process took a little longer.
It's unlikely to take that long - this time of year, with no backlog, non-priority applications usually take 1-2 weeks, maybe 3 weeks.
It sounds like MPI got caught up in the massive backlogs from last summer, which took much longer to clear than normal.
Usually processing times in October/November and Jan/Feb are about 1-3 weeks for non-priority. Then they increase to about 3-4 weeks for applications mailed around March/April, and then much longer processing times, 2-3 months in the summer (May/June to around Sept).
I was just worried because I saw someone who submitted around the same time, for the same visa, using non priority and she has gotten her decision email.
You may well receive your visa very soon - as mentioned above 8-12 weeks is abnormally long for non-priority, and usually only happens in the summer months. This time of year, people usually get their visas within a couple of weeks.
If I was to receive my visa by next week, there is no way we will be ready.
Freaking out like normal, over something that we might not even have to worry about. 
Well it all depends whether they honour your travel date or not - they may well make it valid from April 1st to April 30th, but they might make it valid from, say, mid-Feb to mid-March instead.
You'll just have to see.
If you really cannot travel in the dates they give you, you can apply for a new visa vignette with a new 30-day window, but it means sending off your passport to UKVI again and paying £189 for it.