If I understood well, you already sculpted and molded and now you want it to fit closely on your head?
What is usually done is using a head form (could be your lifecast) that ideally has also the shoulders there and reference keys. And you sculpt on top of that. Then you mold your sculpt and the lifecast/manequin (that has it´s refference keys) becomes part of the mold, this is, the core.
After you open the mold and take/clean the clay out you will have a three part mold (if you molded the sculpt in two parts). Front part negative+back part negative+core (head).
Once closed the standard would be injecting the silicone filling the cavity between the core and the negative parts of the mold. This would be done (injecting, if the core is hollow) through the interior of the core´s head where previously one would have created an injection hole.
If one is working with a full core with no shoulders it can also poured through the neck area once everything is closed. I can also be poured through a pouring hole as described when injecting.
This is the way that everything fits in place (eyes and mouth) and snugs tight to your face.
Now, if I understood well you skipped this part. Anything you do now will be quite random. You can try what you say but:
If you fill the front and place your lifecast in most probably you will get a huge overflow of silicone out of the mold.
Placing the lifecast won´t be easy, you want at least 4mm silicone thickness in a mask. You might have to work out some kind of rigging for the lifecast and the mold so it holds in place exactly where you want. And then fill.
Another option, taking in account at what stage you are now, is using thickeners. It could make things easier. Brushing a thick layer and then placing the lifecast in. But the downside is that thickeners compromise seriously silicone properties. You will loose stretchiness and tear strength .
Also, in any of these cases, make the back part when the silicone is not totally cured as silicone won´t bond to itself that easy once cured.
Slushing silicone doesn´t sound good to me. Remember you need a minimum thickness and that silicone is not latex, it takes a time to cure and it´s not intended for rotococasting. Not saying it can´t be done, but not sure what will come out.
The standard way for silicone maskmaking is more or less what I tried to describe in the first part of this answer.
As for heavy parts, yes, silicone is heavy and it could cause sagging or similar issues. Some mask makers use Soma Foama (silicone foam) to fill heavy parts. Not easy to do.