Who proposed the XBT ticker for Bitcoin and why is it used over the more logical XBC ticker? Some standard requires tickers for currencies to start with an X. So, the usual abbreviation of Bitcoin, BTC, cannot be used in contexts where this standard is applied.

Usually, the unofficial ticker/abbreviation XBT is used in those contexts. Why? Bitcoin consists of two words, bit and coin, it's logical to abbreviate that to BC. For this reason XBC sounds like a much more logical ticker than XBT.

What is the first reference of XBT and is a consideration made before using it that favoured it above XBC?
XBT is used because the ISO 4217 (the standard for currency codes) alphabetic code XBC is already in use.  It stands for the European Unit of Account.

Here's the Official table of ISO 4217 codes

The X indicates it's not a national currency, but rather a supranational currency (like the East Caribbean Dollar, XCD) or something else that isn't quite a currency, like IMF special drawing rights (XDR) or precious metals (XAU is gold, XAG is silver, etc.) for which exchange rates are of interest.

X is used for this because ISO 3166, the standard for country codes, never uses X as the first letter and the currency code is the 2 letter code and usually the first letter of the currency name.  For example, JP is the code for Japan, so the code for the Yen is JPY.  An exception example is the Mexican Peso, code MXN.  MX is the code for Mexico, and the N stands for New Peso.