[SalesForce] Does making a field readonly on pagelayout still show in Edit mode

I know you said no validation rules in your question, but based on your comments it seems like it's more of the aesthetics that's keeping you away from them. From personal experience, as long as you have a good error explanation that doesn't leave users confused, they have no problem with validation rules. In case you decide to go this route, here's a rule that may work for you.

AND($Profile.Name = 'Read-Only Profile', ISCHANGED(Lead_Status))

Error Message: Lead Status is a read-only field for users with the profile 'Read-Only Profile'

Hope this helps if you decide to use this.

Setting 'required' at the sobject's definition level is the 'nuclear' option for requiring data entry as it overrides any field level security you might have. This results in some interesting interactions with High Volume Customer Portals/Customer Communities (If you make the lookup field you are using for record sharing required then the records visibility can be changed by someone who changes the value of this lookup field).

The softer way is a validation rule and marking it as required on page layouts and on VF pages. I would back the 'required' flag off the sObject and do that.