Firstly, Lafuma bags are notorious for being generously rated; I wouldn't be surprised if the +45 degree bag is more like a +50/+55 degree bag. Secondly I do not put much stock in the stated warmth of bag liners-- perhaps a couple (1-3) of degrees, but certainly not "5 or 10".

Lastly, warmer sleeping bags do not need to be heavier and I wouldn't be surprised if all the extra clothes you bring doesn't weigh more than less clothes and a good quality warmer bag-- and lets face it any summer bag is not necessarily considered a "warm" bag and also how much clothing does one really need to bring for summer camping; but I digress.

IMHO, I wouldn't like to be in a +45 degree Lafuma bag in below freezing temps.

Anyway to answer your questions:

#1. Most "good" bivies tend to be waterproof on the bottom and breathable on the top-- the top doesn't need to be waterproof since the tarp, correctly pitched in a carefully chosen location should negate the need for a completely WP bivy. I use a one from TGoat, weighs in at around 8oz and costs about $120.

It should be noted here that the mesh opening is for bug protection and I do not know what you aim to achieve by rigging your poncho over it-- in fact that would be counterproductive the problem you addressed below.

#2. Condensation is pretty much a way of life with tarps/bivies and single wall shelters. Secondly condensation is inevitable in completely WP bivies-- hence why most opt for a breathable top shell. Obviously ventilation and site selection has goes a little way to reduce condensation