If the statements in the question are all accurate, then there is no copyright infringement here. But note that statement 7:
App b copied all 10 features from app a, but expressed all features in a different way where there is no substantial similarity.
includes a legal conclusion, and often a key point in issue in a copyright case. Whether there is or is not "substantial similarity" is ultimately a question for the court to decide. A court may find "substantial similarity" even in the clear absence of source-code copying, provided that the display and interface are sufficiently original to warrant copyright protection.
Indeed since substantial similarity is an essential element of copyright infringement, statement 7 is legally equivalent to saying that "app b does not infringe on app a."
In *Arica Institute, Inc. v. Helen Palmer and Harper & Row Publishers, Incorporated, 970 F.2d 1067 (2d Cir. 1992) the second circuit court of appeals wrote:
Since direct evidence of copying is rare, a court may infer it upon a showing that defendant had access to the copyrighted work, and that the allegedly infringing material bears a substantial similarity to copyrightable elements of plaintiff's work. Folio Impressions, Inc. v. Byer California, 937 F.2d 759, 765 (2d Cir. 1991); Warner Bros., Inc. v. Am. Broadcasting Cos., 654 F.2d 204, 207 (2d Cir. 1981); 3 Nimmer, Nimmer on Copyright § 13.01 [B] at 13-8 n. 26.3. Two works are substantially similar where "the ordinary observer, unless he set out to detect the disparities, would be disposed to overlook them, and regard [the] aesthetic appeal [of the two works] as the same," Peter Pan Fabrics, Inc. v. Martin Weiner Corp., 274 F.2d 487, 489 (2d Cir. 1960). Accordingly, summary judgment may be appropriate "either because the similarity between the two works concerns only 'non-copyrightable elements of the plaintiff's work,' or because no reasonable jury, properly instructed, could find that the two works are substantially similar." Warner Bros. Inc. v. Am. Broadcasting Cos., 720 F.2d 231, 240 (2d Cir. 1983) (citations omitted); Hoehling v. Universal City Studios, Inc., 618 F.2d 972, 977 (2d Cir.), cert. den. 449 U.S. 841, 101 S. Ct. 121, 66 L. Ed. 2d 49 (1980).
In the Arica case the Second Circuit Court did find substantial similarity of certain aspects of the alleged source work, and thus that some copying had occurred that was sufficiently extensive to constitute infringement. But it found that many of those aspects were factual in nature, and thus not copyrightable. The remaining aspects, court found, were a valid fair use of the original.
In Computer Associates International, Inc. v. Altai, Inc., 982 F.2d 693 (2d Cir. 1992) the US Second Circuit Court found that non-literal elements of software, including the structure, sequence and organization of the software, could be protected by copyright, and devised the Abstraction-Filtration-Comparison test. This three step test, as the Wikipedia article about it states, "has been widely adopted by United States courts and recognized by courts outside the United States as well."
Recent copyright cases have tended not to find infringement where similar look-and-feel are produced by significantly different underlying code, particularly where a clean-room design method has been used. But look-and-feel, or interface, where significantly original, may still be protected.
Thus in the case described in the question, one would need to apply the Abstraction-Filtration-Comparison test. One would also need to consider if a significantly original interface had been copied, even if differently implemented. Such analysis can be complex and fact-driven, perhaps more so than the OP realizes.