Peace,
Unfortunatly I do not belive Muhammad to necessarily be the last Messenger of God. My mind is not made up in case there would be another.
I am suspecting the understanding of the phrase which locks the possiblity of more Messengers is wrong. Additionally, why would God send a Messenger of God regularly over centuries up to 600s AD and then suddenly no Messenger shows up again?
It has been a long period without Messengers, but I am sure there will be more Messengers coming around, unless it is time for the end of days soon. Maybe not script-bearing, but propagating God alone and warnings to disbelievers, and exposing the lies of the hypocrites. Much like Jesus did.
As with many other Messengers he would probably be disbelieved in by many if he turned up today. Perhaps there have been Messengers after Quran came in 7th century AD which were anonymous. Some people believe that Rashad Khalifa was a Messenger, however this person may not really have been a role model, but he advocated the use of Quran alone. RK claimed that he was "contacted" by God on several occasions. He also died 19 years after the vision on his pilgrimage and he was born the 19th day of a month.
We need to remember the Messengers bring news, highlights and new Knowledge, but of course they should not bring falsehood and it seems Rashad did that. But he was also an inspiration for many people. It was due to him that I quickly found out the blasphemies of traditional islam. It seems that he truly was a Messenger which went astray, the Quran seems to talk about that possibility when it comes to Messengers as well. Had Dr. Khalifa been more strategic and not ventured into the wealth and fame life he may have had a bigger impact and positive impact than he had, but his life has still made a difference for the world.
If you study RK's life you will see that his blaspemies became worse and worse over the years and it was in the end that he claimed Messengership. Soon thereafter he was murdered. The first edition of his Quran translation was not as distorted as the most available one today, because if you did not know he made more than one edition. The appendices were later inventions while he was on a living spree, it seems he derailed and went on his own path.
Regarding errors in the so-called code 19; many of them appeared when he became obcessed with this code and in the beginning they were not so daring and he was more careful. Furthermore, he was only human so certainly what he did contained errors. He most likely also came up with combinations of 19 in his delusions and destroyed the public significance of it. Like some critics of the code claim; "the number 19 is just a number", and yes that is true, so many combinations of it means nothing. It depends on in which context it is applied and for which purpose. The whole "phenomenon" is a sign and not something to derive interpretational ideas from.
There are so much material with code 19 on the Internet and much is wrong, but on the whole the code is true in its existence. Please study some simple counts of 19 in Quran such as 'Basmalah' and some people present material which speaks for itself. I know some people call it stupid to believe in 19, but I did from the first time I saw it and I still do. I do not aim at proving it for there seems not to be any point in doing so and many people have done it but it is not believed in. Proving code 19 is like proving God's existence.
God bless you