The dataset is small by AV standards, but we aren't an AV company. We can only use as much as real AV companies are willing to share with us. If you'd like to share more, we would be happy to take it :)
The model is fairly robust to new data, and we tested it with malware from a completely separate source than our training data - so there shouldn't be any share items like icons between the training set and the 2nd testing set. However, we aren't arguing that is of an AV quality today. The main purpose of this research was to get a neural network to train on this kind of data at all, as it is non trivial and common tools (like batch-norm) didn't translate to this problem space.
We are looking at the modification issue! I can't share any results yet since we have to go through pre-publication review, but the issue isn't unknown to us!