The pitfalls are numerous.Wills need to be worded very carefully and their meaning must be clear without doubt. You may read the will you have written and every thing in it may be crystal clear and unassailable to you, but someone else could see an entirely different meaning in what you have written.
If you use the charity route, the charity will not write the will, a solicitor, who has volunteered to write wills for no cost for anyone who leaves money to the charity in the will, will write it.
I would never contemplate writing the simplest of wills, without using a solicitor.
As for when there is 2 or may be 3, 4 or more wills. A properly drawn up will, will always revoke all previous wills, so problem solved, previous wills do not count.
Read Middlemarch by George Eliot, to see what can happen when there are many wills.