I don't think using a skratti for manual search is a waste, you only need to do it to get 1 or 2 gem income, then set him back researching until you have conj 3, whereupon he casts Apsu.
Likewise I see no issue delaying construction. By running a heavy bless your woodsmen are very effectively early. And jarls are pretty effective just with the bless itself as minor thugs - your prophet skratti certainly can be. This way once you have the construction research you also have the gems to build with. Those levels of research also allow you to support with mages in combat, in particular Alteration, quickness and body ethereal make your giants substantially better
1 - I'm afg, roughly I think I have, in CBM, an imprisoned Earth Mother E9N8, Dom 6, Order 2 Sloth 3 Cold 3 Growth 1 Misfortune 1 Magic 1. That could be off slightly on either misfortune or sloth
2 - Not sure exactly, would have to check in game.. probably around the late teens, depending how aggressively you site search vs research
3 - This is situational.. for raiding purposes, against PD, an individual thug or two would suffice. In huge battles you are better off mixing with troops and mages. For medium sized battles somewhere in between.. the philosophy I've taken with sloth 3 is that I'll be relying quite heavily on thugs/mages once I'm big enough for my cap only woodsmen not to be the mainstay of my forces. Certainly given the high regen, especially skratti with a shroud, it is worth having your thugs in groups or with troops, to avoid damage getting focused, so that your regen can do its thing.
4 - Again, situational, depending on strategic needs and how much cash you are gaining from expansion. Once you can build your dom limit of woodsmen plus a norna each turn, save the excess until you can build a castle, and get cracking. It should happen within the first year. Alternately if you find a good rush target, let their capital be your second fort
In both my current games I've started attacking someone before a fort is complete, and have spent spare cash on PD and indies to support the invasion. In one of them I have spent a heap of cash on amphibious and subsequently aquatic troops, starting adjacent to an inland sea bordered by no aquatic nations.