A Dead Certainty
by Kegler

Ten of the answers must be modified in order to fit into the diagram. It's inevitable that solvers will deduce the two-word phrase those alterations suggest. Answers include five proper nouns.

A Dead Certainty grid

Across
1Car rattled; it backed up crazily at first, confusing cleric in northern region (2 wds)
7Thus, pet returns for Mexican food
10Trademark protects her measure of heat
12Favor other holdings for type of IRA
14Expand about spiritual guide
15Desert the French way
16Operating system from Los Angeles in part of Asia?
17Greeting to continue involving Yoko getting crazier (2 wds)
18One getting married, surprisingly, in cafe
21Tom nearly follows a large singer
23Regarding barrier, one might take this?
24Unpleasant person, one with title, returning after starting to cry
26An insane groan disturbed those in the 90's
28Gets outscored in close series
30Obliterate half of arch, forming a ring
32Cathy crashed fancy boat
33Overgrown, like some Presidents?
35Strong desire to study “Weird Science” following award
36Tailless ermine in portico
37Starts to visualize extremely large developments taking grassland
38Disparage first of patrons to drink noisily
40It's part of the foot, Chief
41I almost did, returning to one with no sense...
42...and not nearly the direction
43In a deliberately obscure way, I call ill type “disoriented”
Down
2Hears cuckoo birds
3Start chewing toffee (2 wds)
4I am getting older making a picture
5Handgun is fishing equipment?
6Hear flub earlier in a poem
7Old Mexican octet milling around lady
8Rachel's confused man
9Witnessed part of play in audition
10Riotously, Greek character starting to laugh, wearing ballet outfit; lousy, lousy
11Credit fool with cape for lack of dignity
13Rewinds half of videos; in one, draws goddesses
19Cutting star running back
20Doctor O. on “Sunset” features people of mixed heritage
22Transient starting to notice one empty net surrounding each man
25One lad excitedly took interest?
26Disreputable cur also related to kissing
27Receive every other sign, shoe or gift
29Good man with urge to sew
30Frau Elsa harbors “Clan of the Cave Bear” author's family
31Stony drunk greeting club's leader
34Two commanding officers have a drink
35Hot spot for leaderless group of witches
38That female tennis player doesn't start
39Official is awake around start of match

Solution