Hi and welcome to the Chiari forum,
First you will need more testing b4 you can decide if you want surgery.....A full spine MRI to rue out disk issues and syringomyelia......and a CINE MRI to see if you have a CSF obstruction.....
Surgery is done to restore CSF flow....and correct any other physical issues such as a retroflexed odontoid....kinked brain stem...etc....
A decompression may not be helpful if you do not have a CSF obstruction....if they plan on other surgery do ask what and why .....
We have a very short list of Drs for your country....not sure if the Drs are close you...-
http://www.medhelp.org/posts/Chiari-Malformation/CHIARI-DRS-LIST-Australia/show/1314732
Make sure the Dr is well experienced with Chiari and plans to use oxygen for you post op....and they rule out ALL related conditions B4 surgery.....
Perm nerve damage can occur if you have a syrinx and it is left untreated too long....Chiari itself can cause issues as well depending on how it is affecting you....we all have different symptoms and effects from this condition...so you will need more testing to know what benefits surgery will offer you.
Yes, yes, and yes to your speech questions.brain fog or lack of concentration yes and YES to fatigue....unrefreshed sleep is one of the top issues for most of us and one reason a sleep study should be done to rule out sleep apnea....