saw a video clip of J. Glenn entering Friendship 7. It was too cumbersome to get inside. Capsule door Was closed externally by some other people. Was it possible for J. Glenn, to open the mercury capsule Friendship VII from inside?
Yes, and non explosively at that.
A "forward pressure hatch" was provided for unaided, albeit slow access by the crewmember.
Doesn't look easy though.
Only used on a real mission once, by Carpenter.
It was via an "Explosive Hatch". This design was first used with the second crewed Mercury Flight, Alan Shepard didn't have such an ability, although he did have a manual latch. The explosive hatch weighed less, and thus was preferred for the orbital missions.