Any chance the BTC mining collective would simply block XCP (and MSC) transactions altogether, making the whole protocol useless?
High chance.
What would be their motivation to block these transactions?
The more transactions are in the block, the more data miner must submit to network. Due to network propagation, if some other miner
mines block with much less transactions in almost the same moment his block will most likely propagate much faster and thus orphan
block first miner mined, one with many transactions. In other words, increased number of transactions will
never be cheered by miners.
Some might do it simply because they can and that is perfectly fine.
Then again that is weighed by the reward of generating 0.0001 additional BTC in revenue (that is why transactions have fees, anyways). Depending on the price of bitcoin in the future, that could be quite significant.
Transaction fees already make up something like 1% of block revenue, and this number will only go up.
https://blockchain.info/charts/transaction-fees-usd?timespan=all&showDataPoints=false&daysAverageString=1&show_header=true&scale=0&address=Plus, XCP and MSC transactions generate a LOT of fees (relatively). Study blockchain.info to see why.