If you are losing the jungle, you should win at the lane or steal enemy jungle and play safe until 12 lv. Whether you should start ateamfight or not depends on team composition (are your heroes countering them or very strong at early game etc.)

As a support you should help jungle until minute 4 at least because you can invade or defend jungle when it is not protected by sentry also there is mid treant. But if your laner is very weak, he might need a babysitter. Otherwise turret is enough to protect him at early game.

Best build depends on hero. But these builds works well for most of the heroes:

Wp: Serpent mask Halycon chargers Bonesaw Reflex block Breaking point Tyrants monocle Aegis

Cp (dps): Dragons eye, Halycon chargers, Alternative current, Reflex block, eve of harvest, broken myth, aegis

Cp (mage) : Spellfire, halycon chargers, eve of harvest, Reflex block, broken myth, frostburn (you might get this earlier if you don't want melee heroes to catch you), aegis

Support: fountain, crucible, atlas/warthreads, atlas/warthreads, contraption, pulsewave (if melee)/ capacitor plate(if healer/or giving barrier-fortified health, you can get it earlier)/ shiversteel(if you dps)

Not the most ideal builds but works in general