Sedenu (pictured left), 26, will add strength at centre-back, with higher-level experience at both Whitstable Town, and most recently, Hollands & Blair in the SCEFL Premier, where he made 33 appearances last season.
Winger Haylock-Ashdown (20) meanwhile, signs on dual forms with Ashford United U23's, having also previously played at Kent County League side, Ashford FC.
Kennington boss, Dan Scorer, has moved to strengthen both ends of the field following Saturday's 4-3 reverse at SC Thamesmead, in looking ahead to the local derby at Lydd Town on Wednesday (Oct 2), and the club's historic FA Vase first round tie against Crowborough Athletic at Homelands on Sunday, October 13.
Of the signings, Scorer said: "We've needed to bolster our options at centre-half since the season started. We have two talented lads in that position in Tommy Penfold and Adam Phillips, but it's a lot of pressure on them to perform week-in-week-out, and felt we needed to add depth in that role for the course of the season.
"Tommy's a young lad and is still learning, so hopefully with less pressure he can enjoy his football and continue to improve.
"Adam has been exceptional for us in recent weeks but sometimes struggles with work commitments, so with Calvin coming in we have a really good balance of options.
"James is a player I've been watching for a season or two now, and every time I see him play he excites me. He'll fit in really well and offers something different for us in attack."
Kennington are without a fixture tomorrow (Sat), with their scheduled Kent Senior Trophy tie against St Peter cancelled, after the Jersey-based side withdrew from the competition.