1 or 3 seems like the easiest most straight forward solution. XT-1 is a specialized liquid with a way lower freezing point than you'll probably need. Just go with X1, it already has biocides in it but I'd add a res plug anyway. However that's just my paranoia.
CFM is nothing without static pressure, for example, if two fans can produce the same CFM say 70CFM but one fan has static pressure of 2.7 and the other has static pressure of 1.4 then the fan with the higher static pressure will be able to carry the CFM further into the case and over the components thus giving great cooling performance and pressure.
You are correct in saying you should never have a filter on the exhaust fans but you need them for the intake fans or otherwise the dust will still get in your case..
Don't do a thinner radiator than 34mm and yes, the ST30 is not my first choice for cooling but it has gotten good reviews. XSPC is my company for radiators as i find their radiators are priced right and perform as well as the more expensive radiators and i am sure you could do a XSPC ex360 radiator on top or maybe even a XT45 360mm radiator with three fans in either push or pull as i think the mounting holes are offset to allow for motherboard connectors such as the auxiliary eight pin power.
This is what i would do, 3xAkasa Apaches 140mm, 2 in front and one on the bottom all with filters and 4xAkasa Apaches, 3 on your radiator on top in pull or push and one in the back sending air out the back without filters.
Good to know that! i'll be building a system myself too in a couple of weeks and i've been roaming around these parts for some time now for some tips and suggestions. Thanks for the info on that