Hi Sara,
You've come so far on your plan and you have done very well so to continue with your plan would be ideal.
There is always a HOWEVER, however. You are getting the jitters and doubting the plan you have followed for so long. This is normal, however those of us who have done a few Marathons and trained for them over long periods, get into the habit of "listening to our bodies". You have done the Spen 20 with an encouraging result, and thankfully it was just long enough before the Marathon not to affect it. It shows that you will complete the Marathon.
If the article you read was a good one it may have warned you that the first 20 miles is for some people only half way. The last 6 is untrained territory. You can pay for starting off too fast. I personally think that you should follow the program as closely as possible and do the 21 miles, and the rest of it. You will then be in the right frame of mind and full of confidence.
But it is your body, you have the feelings, and this is where the caution comes in. If at all you think the remaining training is going to strain/injure you in any way just back off a bit. If you do the 21 can you choose a route where you could cut off for home if you need to and say do 18/19? Beware as the temptation to cut off even if you are going well will be great. This run should be done in a relaxed manner (and not so many Spen-type hills) at a steady pace throughout preferably not actually "checking" your pace to see if you are going "fast" enough.
It is these runs which give you your endurance and confidence. As for the 10-miler the week before, that would be fine, no more, and again try and and do this in a relaxed manner in the morning with your mind one week ahead.
You mentioned the snow, of course it is not helping us and that is more likely to cause injury than owt else. If you could travel to East Yorkshire/Yorkish or at least east of the A1 while it is like this to do your long runs, it is not so bad there.
Good luck
P.S. We all respond to different training, and different advice will follow, hopefully from others in the club. Just read it all Sara and try and do what's right for you. Above all, just don't try and overstrain in the last few weeks and don't go the other way and shut right down.
Then when you have completed the Manchester Marathon successfully as I know you will do, lend me that plan