How stringently is the Melava Malka meal to be taken into consideration?
For example, if it would cause a great deal of Bittul Torah, would one still need to fulfill the meal? (which is not the case for a meal of Shabbos, in comparison).
R Ari Enkin brings a number of sources on the topic here. He writes it is a mitzva worth every effort but ultimately optional.
It is considered a great mitzva to prepare and partake of a special meal following the conclusion of Shabbat. [...]
The melave malka meal should preferably be held within fours hours following the conclusion of Shabbat though it may be held any time until midnight. Under extenuating circumstances one can still fulfill the mitzva of eating the melave malka meal until dawn Sunday morning. Although one may not have an appetite to eat yet another meal following the conclusion of a gastronomically filled Shabbat one should nevertheless make every effort to do so.
Ultimately however, partaking of the melave malka meal is essentially optional and not an outright obligation (Shulchan Aruch Harav 300:1, Mishna Berura 300:2).
Some authorities say that if need be one can discharge the melave malka ‘”meal”‘ with merely a hot drink (Piskei Teshuvot 300:1).