in some masajid for jumu'ah people stand directly on right and left of imam, while imam is one step forward. This is due to lack of space- but sometimes there are empty spaces in back areas that may or may not fill up with people eventually. & also sometimes imam wont step forward. Are these valid?

Ustadh AbdulAziz al-Haqqan

Yes, the prayer is valid, whether the imam steps forward or not, as long as no one stands ahead of him, but if there is space behind then the best is that the imam stands alone and they fill the rows behind him.