This was a relatively straightforward puzzle with an easy to find theme since it was clearly indicated by the extra letters in the wordplay and the ‘three components’ in ‘appropriate relative positions’ could be quickly spotted.
The extra letters in the wordplay spelt out MAN’S NAME, GEOGRAPHICAL FEATURE and MEANS OF CROSSING and a quick check on-line soon confirmed that on 30th June 1859 Charles Blondin crossed Niagara Falls on a tightrope for the first time.
Before highlighting, 7a needed to be changed to LESLEY, 23d to RETIAL and 32d to ARSEY. CHARLES BLONDIN (1st & 3rd rows), TIGHTROPE (6th row) and NIAGARA FALLS (10th & 11th rows) could then be highlighted in accordance with the preamble.
I’m not sure that the need to change three letters in the grid added anything to the puzzle, the amended entries could have been clued just as easily and it certainly didn’t provide any disguise for the words to be highlighted.
*(XXX) – anagram
[x] – letter(s) removed/unused
{x} – extra letters in the wordplay
Across
2 ESCHAR CH (champion) in *(S{m}EAR)
7 LESLEY before alteration WESLEY S (Sweden) in *(LEEW{a}Y)
12 RACA RA (god) CA{n} (John)
13 UNENVYING *(E NUN{s}) VYING (making a declaration)
15 ASH-BLOND {n}ASH (English dandy) L (fifty) in BOND (chemical attraction)
17 WAUL W (women) AUL{a} (hall)
18 ACCITE ACC (accompanied) {m}ITE (small childe)
20 STEATOSIS *([p]OTTS [d]IS{e}ASE)
21 UPTIGHT PU{g} (boxer) reversed TIGHT (under control)
25 BEFORE B{e}EF (argument) ORE (tangle)
27 ERASER *(SEA-R{o}[v]ER)
29 ATHLONE A (one) TH (Thursday) L (lecturer) {g}ONE (disappeared)
31 CATATONIA *({r}ATIONA[l] ACT)
34 HERNIA HE{a}R (TRY) [a]N[t]I [l]A[x]
35 AGAS A GA{p} (breach of continuity) S (succeeded)
36 ALLSPICE *(C{h}IPS) in [g]ALLE[y] (middle of ship’s kitchen)
37 ON THE NOSE ON (drunk) *(HET) NO{i}SE (din)
38 HEAR E (energy) in {c}HAR (cleaner)
39 DESERT E (earth) in *(TRE{a}DS)
40 SLYEST S (soprano) *(TE{l}LYS)
Down
1 DRAW-TUBE *(DWAR{f}) TUB (shaft casing) E (electronic)
3 SCHUYT Y (yard) in SCH (school) UT{e} (pick-up in Perth)
4 CABLE CAB{a}L (plotters) E (Spain)
5 HULES {t}HULE (northernmost land) S (has)
6 RENNET N{u}N (pigeon, blue tit or duck) in *(TREE)
8 EVICTOR E{r} (King Edward) VICTOR (V)
9 SYNCOPAL *(SPAC{e} ONLY)
10 LIAISE LIA{m} (Irishman) IS (lives) E (European)
11 YGOE GO (work) in {e}YE (mine entrance)
14 NETIZENS NET (effective) I (in) Z{a} (Zambia) ENS (being)
16 OUTHER {n}OUT (Scottish cattle) HER (belonging to a woman)
19 SPELAEAN *(PLEA{s}ES) A N (knight)
22 IOLANTHE I O (duck) L{o}AN (permission to use) THE
23 RETIAL before alteration FETIAL *({f}LAT FI[v]E)
24 DRESSERS DR (doctor) {c}RESSERS (lawyer’s cessations of terms)
26 ROTIFER ROTI (cake) F[ound] E{r}R (stray)
28 STRUTS R (resistance) in ST{o}UT (strong) S (square)
29 ANGLOS A N (new) GLO{s}S (attempt to disguise)
30 ORGIES {s}O (very good) R (rector) GIES (in Scotland provides)
32 ARSEY before alteration ABSEY {i}BSE[n] in AY (always)
33 RAPHE RA{n} (managed) P (positive) HE (male)
34 HOOD H (hydrant) {g}OOD (suitable)