1) how can i stabilize and be more offensive, before enemies gets too strong to defend against--> aforementioned pillow suffocation
It's a long, slow grind. I just won my first 4-player FFA (me vs. 3 comps). I had to target one of them for most of the game, while just holding off the other two, but soon I had two enormous fleets and half the galaxy under my control, with ships to spare to defend random planets (~10 ships to make a mini fleet for defense). Even when the AI used a superweapon (I'm assuming) to destroy an entire fleet (taking me from 0 fleet available to 480, believe it or not), I rebuilt in a matter of 5 to 10 minutes.
Make sure you've got lots of spare money when you start researching higher fleet caps, and spend that money on building ships right away. The fleet number will turn red and go neg, and the ships will be queued up to build right away. I usually wouldn't research a higher fleet cap or capital ship crew number until I had the resources immediately to invest in more capital ships or other ships.
Also, Kodiaks. Massive numbers of Kodiaks with a few flak frigates and other frigates as support. Kodiaks tear up the opponent's caps faster than anything, and they're fast.
2)is the novalith/deathstar weapon worth the investment? can it shoot past more than 1 gravwell?
I usually build two for my endgame, and start taking out colonies. I time them with my fleet so I take out three colonies every five minutes or so. It's great for the endgame.
3) is more than 1 jump inhibitor better/more effective?
I assume the effects scale, increasing jump time by 250% per inhibitor. But I don't know if this means each adds it's effect individually (a 10-second jump becomes a 12.5 second jump with one inhibitor, a 15 second jump with two, etc) or cumulatively (a 10 second jump becomes a 12.5 second jump with one inhibitor, a 13.7 second jump with two, etc). Diminishing returns on phase jump inhibitors doesn't really make sense, though.
4) is planet shield worth it?
I've never used them. I level up the planet's emergency facilities and build a lot of hangars.
Phase inhibitors are great for holding a group of siege frigates in the gravity well so your bombers (from the hangars) can pound on them. They'll usually turn and run after a few minutes, so often I won't even need to send my fleet to save the planet. Or just a few ships (a couple anti-siege frigates, but not the entire fleet).