I think it is correct to stop the custom that people write letters at the siyum sefer tora.
Only the sofer should write all the letters and complete finishing the sefer.
Allowing others to write letters causes several problems:
1) people that are mechalel shabos are honored / they are pasul to write.
2) many people although told, don't mekadesh the csiva, and when there are many people honoured the sofer forgets to remind each and everyone, sometimes the sofer is embarresed to make a issue, so he keeps quite.
Holding the feather by the person honored - thinking the sofer is writing, although correct that it is the sofers csiva - also causes problems, many times he pushes the hand of the sofer causing disturbance in the csiva and tzuras haois.
Leaving hollow letters to my opinion doesn't help at all - since the iykar csiva is the one that finishes the letter, and the begining thin lines is not a kosher letter yet.
Even according to the thought that the thin lines are a letter, the person writing over them is csav al gabei csav - which is problematic if the last one isn't kosher.
Obviously there are situations where everything is 100% but the custom is copied everywhere getting into very touchy [touching somebody's honor] situation.
Chazal said: the chacham is one who sees beforehand not to get into a bad/difficult situation.
I truly think it is a big mitzva and cavod for the kedushas sefer tora to be mevatel this custom.