Glendening is a good 4th-liner. You need 4th-liners. Young, has energy, drive, can PK with the best of them, intelligent, a good leader, a pest, has speed to burn, works hard, hits. I really don't see what the issue is here, unless the OP is strictly angling for Ferraro being better. Which, maybe he is, maybe he isn't. But isn't the hope with Ferraro that he'll be good enough to play above the 4th line?
Ideally the top 3 lines are centered by a combination of Z, D, Sheahan, Weiss, Legwand.
The fourth line is centered by Helm, who brings far more than Glendening. I can't believe I'm saying this, but in comparison to Glendening, Helm has a scoring touch. He's way faster as well. The only thing Glendening brings that Helm doesn't is that agitator side, but I put way less value on that compared to speed and scoring. Depth scoring is how you win games.
I don't know if I'd rather have Andersson over Glendening. The speed difference is huge, but from what I've seen in the games, Andersson has the advantage in faceoffs. Which is a pretty key part of being a centerman. Considering they fill the same role and would only see limited 4th line minutes anyway, I think it doesn't matter too much if which one plays there.
Versus Helm though? Both of those guys sit for Helm.
Glendening can play any forward position.
He has only shown a 4th line skillset so far, and if he's not playing center that leaves 2 wing spots.
I guess this is an answer to your second part, but I fill out the top9 with guys like Z, D, Franzen, Nyquist, Tatar, Sheahan, Jurco, Weiss, Alfie, Legwand, (MANTHA???) That's plenty. Hell, throw in Abby if you want. Fourth line can be filled with Helm, Miller, (Abby?) Ferraro, Glendening, Andersson, Callahan. At this point, I'd go with Helm, Miller, Abby. I'd go with Glendening over Andersson if we're talking wing. And I'd rather see what Ferraro and Callahan can bring on the fourth line. By most accounts, they play a fast, gritty game
with a bit of a scoring touch.