1) What replaces the Venite on the 19th day of the month in mp? 2) When should the Nicene Creed be used in place of the Apostle's Creed in mp and ep? 3) When should Jubilate Deo and Deus Misereatur be used in mp and ep?
As far as I can make out, nothing, because Psalm 95 is one of the psalms for the day. e.g. Ps.95, 96, 97, 98, 99, 100, 101. We should presumably all be satisfied with just those. It shouldn't, the only alternative suggested is The Creed of St Athanasius, on the days appointed. Jubilate is only ever used in Morning Prayer not Evensong, unless appointed as one of the Psalms for the day, i.e Day 19, or for Sunday (which it rarely is). Deus Misereatur only at Evensong, unless on the 12th day, which has Ps.65, 66 and 67, and then only optional. Magnificat and Nunc Dimittis being the most favoured format for Evensong, unless you know a nice setting for the Deus Misereatur. Let's not have any vain repetitions. .
Regarding your response to question 2, there must be a difference between our BCPs in this(England and America) because the Nicene Creed is offered as the only alternative to the Apostle's Creed in 1928 (I don't even think we have the Athanasian creed in the 1792, 1892, or 1928). Question 3 was more about asking when it would be appropriate to use the Deus Misereatur in place of the Nunc dimitis (seasonally). The same for substituting the Benedictus for Jubilate Deo. Some substitutions like the Benedictus Es, Domine are generally only used for particular seasons such as lent.
All these options are really suggestions though. You probably won't be burnt at the stake for saying the wrong one sometimes. .
Another canticle. Or, in certain cases (the 1928 American BCP specificies Ash Wednesday and Good Friday) nothing. However, the 1928 BCP assumed that the majority of congregations were not reading the 30 day Psalter and instead offered proper Psalms to accompany the lectionary. If you follow the suggestions in the lectionary table, you will never read the complete Psalter. When Morning Prayer is used as the principle Sunday service. In the American South, this practice persisted into the 1970s. It still exists in small pockets of some of the characteristically 'low church' Continuing groups.
My copy of the BCP*, with regards to Venite (Psalm 95) at the start of MP is that on Easter Day it is replaced by anthems. On 19th of each month it is omitted at the start MP but it to be read "in the ordinary course of the Psalms". My copy of the BCP*, provides only the Athanasian Creed on certain days in place of the Apostles' Creed at MP and only permits the Apsotles' Creed at EP. My copy of the BCP*, simply says Jubilate Deo may replace Benedictus, but does not prescribe when this may not may not happen. Similarly, Deus Misereatur may replace Nunc Dimittis at EP. The only proviso is that Deus Misereatur may not replace Nunc Dimittis on the 12th day of the month when Deus Misereatur (Psalm 67) is one of the psalms to be said on that day. *My BCP is that of the C of E. I note you are in the USA. Therefore, my answers my only be of interest but may not provide answers to your questions.