In Psalm 119:67, the last word is a Qal perfect first person common singular of the verb "to guard, watch, keep." I'm curious about the qåmeṣ vowel under the mem when I expected a pàṯaḥ. Does this have anything to do with its being the final word in the verse?
I'm unable to answer this as I would like, but will put what I did find.
The other six occurances of שָׁמַ֜רְתִּי (Qal perfect 3rd person singular) 1 Samuel 25:21; 2 Samuel 22:22; Psalm 17:4; Psalm 18:22; Psalm 119:168; and Job 23:11 all have the patah under mem.
Only Psalm 119:67 does not and it is only at the end of the sentence in Psalm 119:67. – שָׁמָֽרְתִּי
Thus, while I cannot validate your suspected conclusion, it is consistent with what is available in the MT.
Something seems to be going on with the perfect tense here. The Jewish Publication Society (JPS) translates it a English present tense.
Before I was humbled I went astray, but now I keep Your word.
Before I was afflicted I went astray, but now I keep your word.
However, Psalm 119:168 is also translated with English present tense, but has the expected vowel points for Hebrew perfect tense.
Here are some possible uses of perfect tense, but it doesn't explain the changed vowel point.
(c) To express actions or facts, which are meant to be indicated as existing in the future in a completed state (futurum exactum), e.g. Is 4:4 אִם רָחַץ when he has washed away=when he shall have washed away (an imperfect follows in the co-ordinate sentence; cf. the conditional sentences in § 107 x); Is 6:11 (after עַד אֲשֶׁר אִם, as in Gn 28:15, Nu 32:17; also 2 S 17:13 after עַד אֲשֶׁר, Gn 24:19 after עַד אִם and elsewhere frequently after temporal conjunctions); Mi 5:2 (יָלָ֑דָה); Gn 43:14 כַּֽאֲשֶׁר שָׁכֹ֫לְתִּי שָׁכָֽלְתִּי וַֽאֲנִי and I—if I am bereaved (orbus fuero), I am bereaved, an expression of despairing resignation. Cf. Pr 23:15, Est 4:16. -- Gesenius, F. W. (1910). Gesenius’ Hebrew grammar. (E. Kautzsch & S. A. E. Cowley, Eds.) (2d English ed., p. 313). Oxford: Clarendon Press.
(b) In a class of actions which are completed just in the act of giving them expression. This usage appears chiefly with verbs denoting to speak, as verbs of swearing, declaring, advising, and the like, or their equivalents in gesture. Deu. 26:3 הִגַּדְתִּי הַוּוֹם I profess this day; 2 S. 17:11 כִּי יָעַצְתִּי I advise; 2 S. 19:8 בַּֽיהוָֹה נִשְׁבַּעְתִּי I swear by the Lord. Gen. 22:16, Nu. 14:20, Deu. 4:26; 26:17; 30:15, 18, 19, 1 S. 17:10, 2 S. 16:4 I worship, 19:30, 1 K. 2:42 I hear (obey), 2 K. 9:3 I anoint, Jer. 22:5; 42:19, Ez. 36:7, Ps. 129:8; 130:1, Pr. 17:5. Song 2:7 I adjure. So the frequent אָמַר י׳ saith Je., or כֹּה א׳ י׳ thus saith. In some cases impf. יֹאמַר is used, hardly as a frequent. but as a present. This occurs in the midst of a speech, Is. 1:11, 18; 33:10; 40:1, 25; 41:21, Ps. 12:6. Both forms Is. 66:9. -- Davidson, A. B. (1902). Introductory Hebrew grammar Hebrew syntax (3d ed., p. 60). Edinburgh: T&T Clark.
The reason for the qamets instead of the patach on שָׁמָֽרְתִּי is the accent silluq, it looks just like metheg, and it will lengthen the short vowel. http://www.pericope.net/Assets/pericope_texts/Pausal_Forms_Revell/PausalTNK.pdf