Interesting. I personally would doubt that Galatians 5:11 is alluding to Acts 16:3 per se since the former text refers to Paul's "preaching" (kérussó) of circumcision, whereas Acts 16:3 makes no reference to such preaching (indeed Acts 21:21 refers to Paul teaching the opposite), and instead refers only to his actions. Nevertheless, I think Acts 16:3 is quite relevant because the incident is located in Lystra in south Galatia (v. 1), and Paul then passed through to north Galatia (v. 6), so the fact that Galatians is also addressed to the Christians of Galatia would appear to merit consideration of the idea that a tradition of Timothy's circumcision or a memory of the actual event lies behind Paul's expression in Galatians 5:11. One possible relation between the tradition/event and the phrasing in Galatians is the possibility that the tradition/memory of Paul's circumcision of Timothy gave rise to a local belief in Galatia that Paul was in favor of circumcising Greeks or half-Greeks (as Timothy was if Acts 16 is to be believed). This would explain why Paul expends so much energy in his apologia in affirming his own position on the matter of the Law and circumcision. In this vein, we might note the connection between Galatians 5:11 and 1:10, where Paul says: "Am I now trying to win the approval of men, or of God? Or am I trying to please men? If I were still trying to please men, I would not be a servant of Christ". This rhetorical form is the same as in 5:11, where an "if ... still" (ei ... eti) clause is used to deny one of the charges made by Paul's opponents. In this case, it would be that Paul formerly tried to please men and win their approval, and this is the sort of thing implied in Acts 16:1-3, that Paul circumcised Timothy in order not to offend the local Galatians. Rather than having Galatians allude directly to the account in Acts, I think it alludes to a wider tradition/memory in the Galatian churches (as employed by the pro-circumcision Judaizers) that has both Paul circumcising Timothy and preaching circumcision. I also think that the use of the same "if ... still" construction in 1:10 suggests that 5:11 is probably not an independent interpolation but likely from the same author as 1:10.
Might it be possible that Paul had a more nuanced position that caused his opponents to accuse him of wishy-washiness -- just as Paul himself accused of Cephas (Galatians 2:11-14). A charge of inconsistency fits well with what is said in the preceding verse to 5:11: "I am confident in the Lord that you will take no other view" (v. 10). The paralleled statement in 1:10 also fits with a charge of inconsistency, of an opportunistic attempt to win the approval of men. In 1 Corinthians 9:20-21, Paul himself referred to his accommodation to observant Jews: "To the Jews I became like a Jew, to win the Jews. To those under the Law I became like one under the Law ..., so as to win those under the Law. To those not having the Law, I became like one not having the Law". His opponents may have well framed the issue thus: Paul says that circumcision does not matter, yet he had Timothy circumcised. If the details about Timothy's parentage were not merely devised to provide a rationale for Paul's actions, Acts 16:1-2 would suggest that Timothy was legally a Jew (his mother being Jewish) and not a Gentile, and thus Paul's circumcision of Timothy was not analogous to that demanded for Titus (cf. Galatians 2:3). Paul's consistent position could have been that he rejected any circumcision for Gentiles, as it would not contribute to their salvation, but for Jews, they should not be uncircumcised (cf. 1 Corinthians 7:18-19), and if not already circumcised, circumcision would not be encouraged unless it would help "win Jews" who would otherwise not listen to a known uncircumcised Jew. Just as Paul allowed the eating of meat sacrificed to idols as long as the person knows and believes that idols are meaningless (1 Corinthians 8:1-10), so he could've provisionally allowed the continued circumcision of Jews as long as they do so not to obey the Law to remain under the Law, but for other reasons -- such as accommodating to the Jews to win them over to Christ (being the politically expedient thing to do). This may be a nuance that could have been lost to the minds of Paul's Jewish-Christian opponents. Of course, this is just a guess. :)
As for Galatians being a response to Acts, the two seem to be pretty independent. Is there any trace of Lukan language in Galatians? I am also very hesitant of a late date for at least the core of Galatians considering the apparent allusions and verbal echoes in 1 Clement, Ignatius, Polycarp, Diognetus, and Justin Martyr (which I probably give more weight than you do).
