Please do a bit more research, before asking so many questions at one time..
A short research about such a trip, will answer many of your questions.
I try to maintain a lîst of all travellers on the west who writes a blog or website. There you will find some recent informations about how others did that trip, which route, which visas, which paperwork:
Trans-Africa: Gerade Unterwegs...
I did collect informations about the Visa Situation at 2013 here:
4x4tripping: Westroute Afrika, aktuelle Visa Situation 2013
Please note that the visa situation changes rapidly - therefore you have to read these most recent transafricas noted in link 1 - to see what are the current rules.
Here at Hubb is a thread who shows recent experiences till Juni 2014 what not mean that you have to check the current situation like mentioned before.
As you will see there, there are mostly severall options to optain the visa for a given country.
The Carnet de Passages is a theme itself. You will find severall threads here at Hubb about people who did it over the westroute down without a carnet and too people who use it.
Here you find some informations and backlinks, use google translate if necessary.
I did like the cashless bordercrossing who comes with the carnet. It costs 220 Euros, is issued inside of a 3 days, but you had to deposit a good amount of money depending on your car or bike. The UK People dont talked very happy about the UK RAC, so you should do a short search about here at HuBB.
Let me pick some of your questions:
a) it is not possible to enter the westroute over Algeria or beyond, because the marocco-Algeria Borders are closed. You had to start with Marocco
b.) start with as many visas you can, let you spending your traveltime for your trip not for paperwork during your journey ;-)
c.) no problems "usually". Which a good load of rain, you will meet problems near anywhere but specially in Nigeria, Cameroon, Congo, Dr Congo Sections.
f.) Look at the Ebola Situation. We did the transafrica this way:
This way you will leave out some Ebola problem areas of westafrica, but watch the situations carefully. Borders can get closed rapidly. Watch this
thread at Hubb at sample, about the Nigeria / Cameroon Border.
While tour operators suspend their sceduled tours to avoid loosing money, you can look for yourself, can too make the best out of your trip if you worst case had to return at a place. Currently there is no verified informations about the mentioned land bordercrossing between nigeria and cameroon for us overlanders.
Look too at the security situation. You have to do the risk calculation by yourself. Read the blogs of the recent travelers, do a google research, get a feeling about.
Back to topic: Generally doing such a transafrica has always some points where you arent able to be absolutely shure about. Shure about if all roads are passable, if the borders are open, if you are able to cross an area like planned (at sample because of a local unrest, virus outbreak, military operations and so on). So beeing flexible and relaxed is helpfully.
Have fun with your detail planning - it is the first part of your trip
Surfy