I am presuming that you are reading the Deluxe version - since the page number for question #1 fits.
1. On p46, in the advanced martial arts - throw description, it is stated : The foe being thrown cannot weigh more than 3 times the Encumbrance capacity of the character performing the throw. If I read correctly the encumbrance limit on page 36, this is twice the character strength value (this should be reminded within the throw paragraph). It means that an average human is limited to throwing a 20-pound foe. Either I understand something wrong or a modification is needed. Maybe you could consider that a character can throw up to his own weight plus his strength value ?
You are thinking "Encumbrance Unit". In short, because I kinda changed things with Encumbrance at one point, this doesn't make total sense. Basically, your Encumbrance Capacity is going to be a multiple of your Encumbrance Unit (i.e. 5x your Encumbrance Unit --- a modifier of -20 from the encumbrance).
Thus, if you have a Strength Stat of 15, that gives you an Encumbrance Unit of 30 lbs. Thus, your encumbrance capacity is (5 x 30) = 150 lbs and 3x that means you can throw a foe who weighs up to 450 lbs (i.e. 3x Encumbrance Capacity).
2. How to handle sneak attacks from behind ? From what I gather from different places in the book : if you succeed to strike after a successful stealth roll, you gain an extra boon point (p23, stealth) and if the target is unaware of the attack, its defense drops to 12+magical bonuses. I suggest that a new special situation be added on page 39 explaining that situation, something like : if the foe is ambushed (caught unaware from behind), his defense drops to 12 and the attacker gains an extra boon point. Maybe it could go along with a position bonus for the attacker, or do you consider that the defense drop is enough of an advantage ?
Note that the target must be caught COMPLETELY unaware for the change in DEF to take effect. In melee combat, that (the reduced DEF) won't happen, even if you are able to get in a sneak attack.
Now, I will be adding in some positional bonuses in the next version - to account for such things.
3. Do armor penalties to speed (p31make ) make speed bonus drop below zero or not ? I think it so. But then I have another question. Does a negative speed modifier makes your defense drops below 15 ? I think it should. But in that case, someone wearing a full plate armor would have a -5 speed penalty. That could make his defense drops to 10, which is less than the defense of a target being caught unaware or immobile (DEF 12, page 16).
Yes, it could -- I have been thinking about having the character's Str bonus reduce the size of the penalty before applying it to the Speed Bonus. Looks like I may need to go ahead and do that.
4. On page 32, it is mentioned that some bracers give a bonus to a character's DEF. I haven't found any bonus anywhere. Or is it a reference to the advantage given to unarmed fighters when fighting defensively versus armed opponents (p39) ?
It is a reference to magical Bracers and/or bracers made of special materials (i.e. Bracers of Defense - page 97).