Is it permissible for someone who doubts the number of prostrations (sujud) but inclines towards having completed them fully to perform the prostration of forgetfulness (sujud al-sahw) after the salam, and does this differ from a case where he inclines towards a deficiency or does not incline towards anything?

Your prayer is valid if you are certain there was no deficiency after the doubt, and your prostration of forgetfulness (sujud al-sahw) is permissible according to some Maliki scholars. The Maliki school's position on the timing of sujud al-sahw is: before the salutation (taslim) for a deficiency, and after it for an addition. If both a deficiency and an addition occur, it is done before the salutation. There is flexibility in this matter. What you did does not fall under an addition, and sujud al-sahw here is to humble Satan. However, if you are not certain about a deficiency, then you must build upon the lesser number (of rak'ahs), and perform sujud al-sahw after the salutation. This applies if the doubt is not chronic; otherwise, you must obligatorily build upon the greater number.