From Pali Text Society’s dictionary.
Antarāya
The Pali Text Society's Pali-English dictionary ([p. 48])(The Pali Text Society's Pali-English dictionary)
Antarāya Antarāya1 [antara + aya from i , lit. “coming in between”] obstacle, hindrance, impediment to ( – ˚); prevention, bar; danger, accident to ( – ). There are 10 dangers (to or from) enumd. at Vin i.112, 169 etc., viz. rāja˚, cora˚, aggi˚, udaka˚, manussa˚, amanussa˚, vāḷa˚, siriŋsapa˚, jīvita˚, brahmacariya˚ . In B.Sk. 7 at Divy 544, viz. rājā – caura – manuṣy – amanuṣya – vyāḍ – agny – udakaŋ. – D i.3, 25, 26; A iii.243, 306; iv.320; Sn 691, 692; Dh 286 (= jīvit˚ DhA iii.431); J i.62, 128; KhA 181; DhA ii 52; VvA 1 = PvA 1 (hat˚ removing the obstacles) – antarāyaŋ karoti to keep away from, hinder, hold back, prevent, destroy Vin i.15; J vi.171; Vism 120; PvA 20.
– kara one who causes impediments or bars the way, an obstructor D i.227; S i.34; A i.161; Pv iv.322
Nīvaraṇa
The Pali Text Society's Pali-English dictionary ([p. 376])(The Pali Text Society's Pali-English dictionary)
Nīvaraṇa Nīvaraṇa (nt. occasionally m.) [Sk. *nivāraṇa, nis+ varaṇa of vṛ (vṛṇoti), see nibbuta & cp. nivāraṇa] an obstacle, hindrance, only as tt. applied to obstacles in an ethical sense & usually enumd or referred to in a set of 5 (as pañca nīvaraṇāni and p. āvaraṇāni), viz. kāmacchanda, (abhijjhā – )vyāpāda, thīna – middha, uddhaccakukkucca, vicikicchā i. e. sensuality, ill – will, torpor of mind or body, worry, wavering (cp. Dhs. trsl. p. 310): D i.73 (˚e, acc. pl.), 246; ii.83, 300; iii.49 sq., 101, 234, 278; S ii.23; iii.149; v.60, 84 sq., 93 sq., 145, 160, 226, 327, 439; M i.60, 144, 276; iii.4, 295; A i.3, 161; iii.16, 63, 230 sq.; 386; iv.457; v.16, 195, 322; Sn 17; Nd1 13; Nd2 379; Ps i.31, 129, 163; Pug 68; Dhs 1059, 1136, 1495; Vbh 199, 244, 378; Nett 11, 13, 94; Vism 146, 189; DA i.213; Sdhp 459, 493 and passim. <-> Other enumns are occasionally found e. g. 10 at S v.110; 8 at M i.360 sq.; 6 at Dhs 1152.
They both mean obstacle or hindrance. So at first pass it doesn’t seem a stretch to view them as synonyms. Will be very interested to hear how their specific usages in the EBT distinguish their meaning.