High Imperial Roman army vs Late Roman army

[Marja]

Do we have any idea what the losses in those wars were?

Not really.

I think Zosimus claims there were enormous losses at Mursa, and the troops transferred from Gallia and lost at Mursa and other battles may have contributed to the collapse of the Rhine frontier, that prompted Constantius II to appoint Iulianus. I just think that the Persian war probably involved worse losses than the Gothic war, and the major civil wars would also be important.

Is there any indication that Stilicho returned the pseudo-comitatenses to border-guard status, or were they just incorporated into the field army?

I don't think all the pseudocomitatenses were the same. Some of the pseudo-comitatenses seem to correspond with border units from territories the Romans had surrendered, particularly to the Persians. Others seem to be local garrisons in the interior rather than mobile units. Some of the pseudo-comitatenses may have been unusual/nonstandard units otherwise equivalent to the comitatenses.