Won't it be wiser to live in a state of harmony with God all the time, instead of projecting it to some end time event we have to prepare for that may or may not come in our life time?

2. Does "no mediator" mean Jesus will no longer be with us? That we have to be "perfect" on our own?

Read the wonderful promises that Jesus promised:
Hebs 13:5 he has said, I will never leave thee, nor forsake thee.
Matt. 28:20 lo, I am with you alway, even unto the end of the world. Amen.

The most important thing is to cling to Jesus in faith and trust and love.
It's the only thing that will get us through. That's true in the relatively smaller trials now, and it will be true when everything goes wrong in the future, if we learn to fully trust Him now.

No mediator, means we need to have all our sins sent to the sanctuary NOW,
and be to be covered with His righteousness.
There will be no turning around once probation is closed.
Don't hang unto ANY cherished sins, settle it now with our wonderful Mediator and be covered by His blood.
Overcome? I've found that the only way to overcome sin, is to walk with One Who is righteous.
Reckon (consider) yourself dead to sin (for Christ was crucified for your sin) and now be alive in Christ. Faith is believing that Christ took your sins and gave you His righteousness, now WALK with Him literally making Him part of everything you do.

Reflect?
What is reflection? There is something greater than the reflection, which is the source of the reflection.
There's only one way to REFLECT the image of Christ --
That is to be walking so close to Him that the light of His character bounces off from you.
It does not mean we become little christs full of our own goodness, no, Christ is always the light, we are always dependent upon Him for light, and by being close to Him and inviting Him deep into our lives, then His light can and will be reflected by us.