After searching and reading various analyses and interpretations of "papañca" and "papañcasaññāsaṅkhā", I'm glad to have found a pretty satisfactory one in the Pali Text Society's Pali-English Dictionary:http://dsalsrv02.uchicago.edu/cgi-bin/p ... ali.719683
Papañca [in its P. meaning uncertain whether identical with Sk. prapañca (pra+pañc to spread out; meaning "expansion, diffuseness, manifoldedness"; cp. papañceti & papañca 3) more likely, as suggested by etym. & meaning of Lat. im -- ped -- iment -- um, connected withpada, thus perhaps originally "pa -- pad -- ya," i. e. what is in front of (i. e. in the way of) the feet (as an obstacle)] 1. obstacle, impediment, a burden which causes delay, hindrance, delay
DhA i.18; ii.91 (kathā˚). ˚ŋ karoti to delay, to tarry J iv.145; ˚ŋ akatvā without delay J i.260;vi.392. -- ati˚ too great a delay J i.64; ii.92. -- 2. illusion, obsession, hindrance to spiritual progress
M i.65; S i.100; iv.52, 71; A ii.161 sq.;iii.393 sq.; Sn 530 (=taṇhā -- diṭṭhi -- mānabheda -- p. SnA 431; and generally in Commentaries so resolved, without verbal analysis); Ud 77 (as f. papañcā); Th 1, 519, 902, 989 (cp. Brethren 344, 345 & J.R.A.S. 1906, 246 sq.; Neumann trsls "Sonderheit," see Lieder p. 210, 211 &Mittlere Sammlung i.119 in trsl. of M i.65 nippapañca); Dh 195, 254 (˚âbhiratā pajā, nippapañcā Tathāgatā; =taṇhādisu p˚ esu abhiratā DhAiii.378); J i.9; Pv iv.134 (=taṇh' -- ādi -- p. PvA 230); Nett 37, 38; SnA 495 (gihi). -- nippapañca (q. v.) without obsession. <-> 3. diffuseness, copiousness
SnA 40. -- sankhā sign or characteristic of obsession Sn 874 (cp. SnA 553; =taṇhā˚ diṭṭhi˚ and māna˚ Nd1 280), 916 (=avijjādayo kilesā mūlaŋ SnA 562). [b]-- saññā (˚sankhā) idea of obsession, idée fixe, illusion[/b] D ii.277 (cp. Dial ii.312); M i.109, 112, 271, 383; S iv.71.
Some other supports for the translation of "saññā" as ideas, thoughts include:
In the same dictionary, for example:
Kāma :t to passion Ud 75;-- saññā lustful idea or thought
Dhamma :asth/s VvA 3, 169;-- saññā righteous thought
In Critical Pali Dictionary: [http://pali.hum.ku.dk/cpd/search.html]
a-kusala-saññā, f., sinful idea
The PTS's translation of "papañcasaññāsaṅkhā" as "idea of obsession, idée fixe, illusion" is close to the Chinese Agamas' translation "諸亂想" [distortional thoughts], "妄想" [delusive thoughts]. Considering the context of MN 18, I'd like to interpret the meaning of "papañca" as "obsessions, delusions, and distortions", and "papañcasaññāsaṅkhā" in MN 18 as distortional, delusive thoughts and ideas of the seven obsessions: sensual desire (kāma-rāgānusaya), aversion (paṭighānusaya), notions (diṭṭhānusaya), doubt (vicikicchānusaya), conceit (mānānusaya), desire to exist (bhava-rāgānusaya), and ignorance (avijjānusaya). The obsessions/hindrances of sensual desire (kāma-rāgānusaya), aversion (paṭighānusaya), desire to exist (bhava-rāgānusaya), and ignorance (avijjānusaya) do not appear to fit in "perceptions and notions ...", or "perceptions and categories ..." or “number of perceptions”, ....
'Bhikkhu, as to the source through which papañcasaññāsaṅkhā (delusive thoughts of the obsessions, e.g. sensual desire) beset a man: if nothing is found there to delight in, welcome, and hold to, this is the end of the underlying tendency to lust, of the underlying tendency to aversion, of the underlying tendency to views (notions), of the underlying tendency to doubt, of the underlying tendency to conceit, of the underlying tendency to desire for being, of the underlying tendency to ignorance; ..."
I tend to think that the other English translations of "papañcasaññāsaṅkhā" don't fit in the following sequence in MN 18 (by going back to perception after thinking):
"With eye & forms as condition, eye-consciousness arises. From the meeting of the three arises contact. With contact as condition, there is feeling. What one feels, one perceives. What one perceives, one thinks about. What one thinks about, one papañceti (becomes obsessed and deluded). When one papañceti (is obsessed and deluded), the papañcasaññāsaṅkhā [distortional, delusive thoughts and ideas of obsessions] assail the one with regard to past, present, & future forms cognizable via the eye...
[ ... perceiving → thinking → papañceti (becoming obsessed and deluded) → papañcasaññāsaṅkhā (distortional, delusive thoughts and ideas of the obsessions)]
Metta to all!