Manfrotto 420NSB Convertible Boom Stand - 12.8' (4m)

The Manfrotto 420B, B&H # MA420B and the 420NSB are the same stand, with the latter not including the empty sandbag.

At max extension this boom is rated for less than 2 lbs. It would not be suitable. The larger Avenger would be able to do it at a partial but not full extension. https://bhpho.to/3bH4yo9

Yes, the legs are 22mm at the ends.

It will manage to hold an apurture of 120 at half the length of the boom

The Manfrotto 208 3/8" Head Mounting Plate with Lock Screws B&H # MA208 can be inserted at the end of the boom in place of the included stud. This plate can then accept a ball head such as the Manfrotto 496RC2 Compact Ball Head with 200PL-14 Quick Release Plate B&H # MA496RC2 to hold your DSLR. https://bhpho.to/2qzA4xA https://bhpho.to/2QuMfg0

When the boom arm is fully extended and the spring lock is pushed downward, the arm can be locking using the main ratchet handle to the left of the spring lock.

The end of the stand will hold a standard Manfrotto stud - some come with 1/4-20 threads so you could attach a mic holder but you would need additional parts. The stand is sturdy enough to hold light heads so it could certainly handle a microphone.

Yes it does have a lever about 2.5 inches long to tighten the boom arm however it does not ratchet.