This can be a rather complicated scenario, and so i would highly appreciate any pointer towards the correct direction.

And So I have setup apache on server A to proxy https traffic το server B, that's a plone site behind varnish and apache.

I connect with A and may see the site on https, things are fine. However, problems start after i upload files, via plone's Publish forms. I'm able to upload small files (~1 Megabytes), however when I attempt to upload a 50MB file, I wait constantly up until the file is submitted, so when the indication is 100%, I recieve a poor gateway (The proxy server received an invalid response from an upstream server.)

It appears in my experience that something timeouts between your communication of the and B and rather than being rerouted towards the correct url, I recieve a poor gateway, as well as the file isn't submitted.

Around the apache log I see

[error] proxy: pass request body failed

As recommended on other threads, I have played around with using the following values without any luck

force-proxy-request-1.0
proxy-nokeepalive
KeepAlive 
KeepAliveTimeout 
proxy-initial-not-pooled
Timeout 
ProxyTimeout 

Sooooo..any suggestions? Thanks millions of ahead of time!