2- Why necessarily does a surviving Byzantine Empire make the trade routes cheaper?
Because of the Ottoman conquest of the Levant, Egypt and most of North Africa in OTL, the Ottomans gained a solid monopoly on the spice trade between the Orient and Europe.
Thus, the Ottomans could (and did) raise the prices of and taxes on goods coming from the Orient.
And Byzantium would not be able to conquer and hold on to all of the Levant, Egypt, and North Africa (at least not during the Medieval Ages and the Renaissance) so it would not be able to gain a monopoly on the trade between Europe and the Orient like the OTL Ottomans did.
3- My reckoning is that the main consequence other the butterfly effect is the end of the Ottoman Empire + butterfly effects
4- Most people on A.H.com know this already, but butterfly effects would be considerable. The Hundred Years War could go either way, no Charles I, probably no Spain, probably no Kalmar Union, HRE decline not inevitable.
I'm not sure about these things, so if anybody knows better on some points could they critique?
The butterfly effect would indeed be considerable, but IMHO the absence of the Ottomans would not even be the largest cause of butterflies.
In a pre-Mantzikert Byzantium survives-scenario, the main cause of major butterflies is the absence of the Crusades.
The OTL struggle of the Byzantines againest the Seljuks in Anatolia would be butterflied away, and with that, Emperor Alexius Comnenus and his letter to the Pope.
And even though the other (direct and indirect) causes of the First Crusade would still exist (like the destruction of the Church of the Church of the Holy Sepulchre at the orders of the Fatimid Caliph al-Hakim, and the frequent Turkmen attacks on Christian pilgrims during the early Seljuk age), Byzantium, which is still the pre-eminent Christian power in the East, would have no reason to want, stimulate, or even tolerate a significant Western European presence in the Levant.
...and the absence of the Crusades is going to cause a whole string of butterflies in Europe (the lives of most late 11th century/early 12th century European kings and princes will be different; people who died IOTL will live ITTL, and many royal marriages will be different from OTL) and the Middle East (no Zengids and therefore no Ayyubids, and either a surviving Fatimid Caliphate or a Seljuk Egypt, to name but a few differences).
The next greatest source of butterflies in this scenario would be the Mongol invasion; the presence of a strong Byzantium (which has an excellent militairy, a long history of dealing with armies of horse archers, and some of the best fortifications in the world) would greatly affect the pattern of the Mongol invasions of both the Middle East as well as Eastern Europe, and it would also affect the dynamics between the *Golden Horde and the *Il-Khanate.
And due to the many butterflies, there's really no telling what happens afterwards, really.