After Vijay collapsed following the dogpile in the first decade, I found myself in such a good position that it seemed a waste to "only" go for the achievement and decided to westernize (originally I wasn't planning to as I figured it'd be easy to take control of India proper without ever having anything to do with the Europeans). Religious ideas were more or less a necessity (Buddhist Ceylon meant every province I took in India had the wrong religion, and in 1.12--I only patched to 1.13 later--I had no easy way of having vassals convert either), which left no room for exploration (the initial plan was to attempt westernization while keeping adm at 5, but I had to revise this later), so I bet on visibility trickling down and jumped my way to the Middle East by means of TCs and Deus Vult.
Much of the land I own in Asia belonged to annexed vassals--most of Indonesia, Malaysia and central India weren't cored by me. This was part of the reason I picked up influence--with religious you save so much bird mana when you're tanking dip that it becomes difficult to leverage it without sufficient dip rep, which is hard to come by early on. While somewhat inefficienct early vassal integrations still help you grow faster. The focus on trade was more or less out of necessity--with so much land (relatively) being overseas and many of my Asian holdings having high autonomy trade income was necessary to fund the growing empire. By the time I'd finished westernizing I'd filled out infuence, but religious came much later (couldn't spare the paper mana for that after getting the initial ideas).
I have no exact figures, but to give you a general idea of the income level in 1563 (when westernization finished) I could swap out the +2 set for +3 and still maintain a max FL consisting mainly of merc infantry (no regulars) and a smaller cavalry contigent and artillery while still making a fair profit every month. So yes, the economy was quite strong at that point. As for manpower, the Ottomans war was the only one that really taxed it outside of the initial expansion, as I made the switch to full merc infantry early on (I'd rather keep worse advisors if I have to than use regular infantry, especially in an area of the world riddled with jungles. You simply lose too much manpower otherwise, even when using your vassals to siege).
No focus for the first few years (I don't like locking myself into a focus before I've a better idea of what my needs are going to be like), but once I had my initial expansion taken care of I went with adm focus, switching over to dip as more and more of the coring (and eventually all) were overseas and bird mana was required for integrations and filling out infuence. As a rule I don't focus mil in SP, unless it's an abysmal start, I'm opting for a military group for whatever reason and/or I'm likely to be stuck with a 0-1 mil leader for a long time.
Overseas coring is hardly an exploit--it might be cheesy to wall off provinces using vassals, but the mechanic is there because Paradox put it in, given the 75% autonomy cap on those provinces. Actual exploits would be things like shortening truces by guaranteeing the tag you just fought and then revoking it (resulting in a unilateral 5-year truce for you instead of a 15-year truce), fully annexing secondary participants in wars to annex them for free (whereas you'd normally be forced to pay bird mana) or hiding all your bird mana in culture conversions after sending a peace offer, potentially allowing you to save 100s of bird mana per war (IIRC they fixed this one, though, but I've never used it--too cheesy even for me).
It's true that attrition losses can be insidious as the way reinforcing works makes it hard to notice that you're actually losing MP (or at the very least gaining less than you could be). I admit I'm somewhat OCD when it comes to that kind of micro in SP, alas--to give you an idea, I'm the kind of player who used to be zealous about ping ponging my diplomats back when they improved relations on arrival. Playing in the HRE I'd spend more time ping poning diplomats than doing anything else... Not too sad they fixed that, heh. If you're playing a lot of MP games your focus will naturally be different, since you can't afford to waste the time on that kind of micro (no one could keep up with it for long).
I rarely focus mil outside of MP--I either go with no focus (pretty much standard opening before I've a better idea of how things are developing around me and what kind of mana numbers I'm likely to have/need in the next few decades) or admin (standard once I think I can get away with it). Do note that I die terribly all the time, but then I imagine that's true for everyone who picks OPMs/small tags and plays aggressively--hell, an early regengy typically means you die to a coalition.
